I shoot a lot of performers in low lighting conditions. Essentially I am looking for a camera that will help me reduce the noise level significantly. I find myself rarely using the kit lenses and mostly using my canon f/1.4 50mm prime which is fantastic in low light and night time but, I do have limitations in spaces where I am too close to the subject(s). This is where my debate comes in about about purchasing a 24-70mm f/2.8L series lens or purchasing the 6D, 7D or 70D plus the 24-70mm f/2.8L lens.
Normally I'd suggest upgrading lenses and not bodies but in this case going to the 6D is going to solve your two problems:
The 6D has significantly better low light performance than the T2i/550D
Your 50mm will have the same field of view on the 6D as a 30mm on the T2i
I recommend you get the 6D and then maybe, if you miss the field of view of a 50mm on a crop sensor the EF-85mm f/1.8
In general, moving from one crop sensor to another isn't going to get you much in the way of low-light performance. Certainly, the 7D has (de facto) exactly the same sensor as the Rebel T2i, and the early indications on the 70D's sensor are that it isn't a significant step up in low-light performance.
Moving to a full-frame sensor would get you one to two stops of low-light performance - but stepping from an f/1.4 lens to an f/2.8 is going to remove two stops.
If the only problem you're experiencing is that you're finding your 50mm f/1.4 too tight for indoor work, it seems to me that you want to get a wider fast prime, rather than anything to do with changing body. Canon have a 24mm f/1.4, a 28mm f/1.8 and a 35mm f/1.4 available, and Sigma's new 18-35mm f/1.8 is also looking like an interesting possibility.
Either the 6D or the lens would work pretty well. It depends on how you want to proceed with upgrading. One big factor with considering the 6D is to remember that it won't work with any EF-s lenses you have, so if you have aspirations to go full frame, beware of that issue.
If you do much shooting other than the low light stuff, the other thing to realize about the 24-70f/2.8 II is that it is the sharpest zoom lens by Canon ever. It takes AMAZING images. It is easily my favorite lens in my kit (slightly beating out my 70-200 f/2.8 IS II) and was the reason I bought the 70-200 because I was so impressed with the 24-70. It is a lens that will last you a long time (probably as long as the EF lens series lasts) and will benefit all your images where you need a zoom in that range.
That said, there is also a lot of room for improvement over both the noise level and general usability of a T2i as well. I personally jumped straight from a XTi to a 5D Mark iii, but I was jumping in to the professional market shooting events for cash bucks. While the 7D has the same sensor and the 70D, while new, sounds like it might not have much better low light performance. The 6D does have a full frame sensor and thus more light hits the sensor. It also has better low light performance. Since it is full frame, it will have a narrower depth of field (which will enhance background blur, but make focusing tighter) it will also reduce the apparent amount of zoom at any given focal length since it isn't cropping anymore.
I don't really think there is a right answer here. It comes down to how much do you look to spend now and in the future and where do you want the gains now vs in the future. Personally, I started investing in optics while I had my xTI to make it less painful when I upgraded, but I also bought the 24-70 f/2.8 at the same time as my 5D.
The other option, as Philip mentions, is to go with an alternate length prime. They won't be as flexible, but the top end primes are still slightly sharper than the 24-70 and let in a lot more light. That can sometimes be a pain for events though. I know I personally prefer to have zoom capability for events.
Either option will serve you well and either option is going to greatly increase the quality of the work you can do. The 24-70 f/2.8 will help increase the quality of your photos whenever you are using a zoom regardless of lighting. The upgraded camera will make better use of the primes you already have and give you more easily accessible controls to shoot faster and more accurately. It's really a toss up based on personal preference and what you think would make you a better shooter based on what gives you the most trouble.
Shooting performers in low light, unless they are perfectly still, means for you aperture is the prime consideration. High ISO/ Low Noise performance is a very close second. To get the best results in this environment you need a wide aperture prime lens and a full frame sensor. In addition to the gain in low light performance the larger sensor gives you, a full frame sensor will increase the angle of view yielded by your 50mm lens, which is currently giving an angle of view when mounted on your T2i (with a 1.6x crop factor) of an 80mm lens on a full frame body.
If you're staying with Canon, that means either the 6D or the 5D mk III. The 5D3 has a focus system equal to the top of the line 1D X, the 6D doesn't. It does take some effort to learn how to use the advanced focus system, but the benefits are well worth the expense and the effort. The 7D has pretty much the exact same sensor as the T2i, and the 70D only shows modest improvement (around 1/3 stop) in terms of S/N ratio, dynamic range, tonal range, etc. Both the 7D and 70D have a mid-level focusing system that is fast and configurable. The 7D is also one of the least consistent focus systems in Canon's current lineup in terms of shot-to-shot accuracy. It is faster and more configurable than the focus system on your T2i, but no more consistently accurate. It does have AF Micro Adjust which will allow you to correct for any back or front focus issues you have, but it will still slightly miss in either direction with about 2-3 times the standard deviation of the 5D3 (See Roger Cicala's blog entry). Based on the early reports I've seen, the 6D falls somewhere between the extremes of the 7D and the 5D3 in terms of focus consistency. I've seen no in depth analysis of the brand new 70D's PDAF system.
In terms of lenses, the EF 24-70mm f/2.8 L II is a remarkable zoom lens. It may be the best AF capable zoom lens ever mass produced. Having said that, the lowly EF 50mm f/1.8 II equals the acutance performance of the 24-70 at 50mm with the apertures they have in common. Your EF 50mm f/1.4 is two stops faster than the 24-70 f/2.8 zooms. If you go to a 6D and the zoom, you'll give back the 1-2 stops you gain in ISO/noise performance to the slower lens. In the not so distant past, most prime lenses were much sharper than even the best zoom lenses. In the current marketplace, the best zooms equal or come close to the sharpness of many primes, but still lose ground in terms of maximum aperture, and to a lessor degree distortion and chromatic aberration.
You haven't mentioned much about how invested you are in the Canon system beyond your T2i, the EF 50mm f/1.4, and the kit lenses you rarely use. You might want to consider other manufacturers along with Canon. In terms of absolute low light performance The Nikon D600 and D800 (along with the pro grade D3s, D4, and Canon 1D X) outperform the 6D and 5D3 by a significant margin.
Like others, I would not go with the 7D, its sensor is not that big of an improvement, if any. I haven't personally tried the 70D, but I don't think it will bring you what you want.
Have you considered getting an EF-S 17-55 F2.8? Its a great lens, and the price has just dropped to under $900. Its wider than your 50 and much faster than the kit, which moves to F5.6 as you zoom out. If you later decide to go Full Frame, the 2.8 keeps its value fairly well.
Another low dollar option is the 40mm F2.8, which is a great prime lens and very inexpensive.