I am struggling to decide between the 5D and the 70D as an upgrade to a 600D. I am an amateur photographer achieving pleasing results with my 600 and I have 5 good lenses. The kit (18-135) is an EF-S so that would have to go if I plump for the 5D. Looking at the features, the 70 seems to offer lots of good upgrades such as 7fps, 20mp, 19 focus points and has the whizzy touch screen and and WiFi capabilities. The fps on the 5 is 1 fps slower and the increase in megapixels (2) is not really much of a differentiator. I tend to use central focus and reframe technique so 61 focus points over 19 - not sure if I should be concerned about that. I favour landscapes and street work over sports and action. Primarily the main difference appears to be that the 5 is full frame over the 70's APS-C however one has to give up some of the niceties such as the WiFi and articulated screen to get there. The money isn't an issue, so from the perspective of being able to produce better photos (assuming that I can develop some talent) I am wondering what more experienced people might think about the choice I am faced with.
If the choice is based over which camera allows me to produce better photos, I'd choose the 5D mark III over the 70D hands down and never look back.
I regularly use both a Canon 7D and a Canon 5D mark II. When I am shooting with only one body it is almost always the Full Frame 5DII. To my eyes there is a visible difference between images made with each body, especially in high contrast/high dynamic range environments and in low light environments. The only times the 7D gets the nod over the 5DII is when I have plenty of light that isn't harsh and I need the extra reach of the 1.6x crop factor or when I need the faster handling of the 7D to catch sports/action. I rarely mount any other lens on the 7D than an EF 70-200mm f/2.8 L IS II. It is one of the best mass produced zoom lenses in the world. I use that lens plus a variety of shorter "L" zooms and non-"L" primes on the 5DII, so the difference in image quality has nothing to do with the quality of the lenses.
The focus system of the 5DII is slower than that of the 7D, but both are about the same in terms of accuracy and shot-to-shot consistency: neither is anywhere near exceptional. In my opinion the two biggest weaknesses of the 7D is the inconsistent focus system and the low light performance in terms of noise. For the 5DII it is only the focus system inconsistency.
For the 5DIII vs. the 70D, the image quality comparisons are very similar to those of the two bodies I use: the FF will noticeably outperform the APS-C sensor in terms of dynamic range, low light performance, tonal range, and color sensitivity. Here's a side-by-side look at the sensor performance of the 5DIII vs. the 70D from DxO Mark. To see the specific results of various tests, click 'Measurements' and then each parameter you wish to compare.
In your case, the difference in terms of handling speed between the 5D mark III and the new 70D is much less than that of the 3.9 fps 5DII and the 8 fps 7D. The other key difference in your comparison is the far superior accuracy and consistency of the 5DIII focus system over the 70D focus system which is the same basic system as the one in the 7D. The focus system in the 5DIII is the same one found in the 1D X. Even when only using the center focus point, you will find that shot-to-shot consistency will be much better with the 5DIII when used with the newer lenses that more precisely communicate with the body how far they have moved. But I would also encourage you to learn to harness the capabilities of that very nice focus system.
It's the good old question. Should I go FX or not? If money isn't an issue then go FX. Better image quality, better low-light performance, far less noise and better video quality (you never know maybe you will also get hooked on video production) and it easier to sync it with strobes so you can use it in a studio. The only downside is the weight and cost of lenses but like you said, money is not an issue. Besides, I think you will definitely develop as a photographer with an FX camera.
The focus points are faster and more accurate in addition to having more of them. The 5D Mark iii has substantially better low light performance and the full frame sensor will result in far better background blur. The battery life is improved and Wi-fi really doesn't matter much. (You can also use an eye-fi card if it really matters to you, which the 5D mark iii supports natively.)
You also pick up improved weather sealing with the 5D Mark iii and easier to use controls. The non-articulating screen is a slight loss, but really, if you want to take photos well, you shouldn't be looking at the screen often. The viewfinder is also 100% coverage on the 5D Mark iii which is nice to have.
As far as value for the money, the 70D and 7D are really nice value, but in terms of quality, the 5D Mark iii simply blows it out of the water (which is why people still buy it despite the much higher cost).
Full declaration, I shoot with a 5D Mark iii.