I am planning a wildlife trip. I want to be able to take photos of far away objects and still have all the detail. I currently have a Canon 650D and my longest lens is 70-300mm. I am busy saving for this trip and a converter or lens. What considerations should I take into account for each option?
Teleconverters tend to be better when used on f/2.8 or faster lenses and on primes vs. zooms. Adding one to a 70-300 consumer-grade zoom (if you don't have the L version of the 70-300) is problematic at best, since most of these are f/5.6 lenses at the long end, and adding even just a 1.4x tc to it makes it an f/8 lens--at which point an entry-level dSLR probably can't reliably autofocus, or at least will autofocus very slowly. Which kinda sucks for wildlife shooting. A 2x tc would make it an f/11 lens and is right out.
In addition to the lower aperture issue, TCs nearly always add some softness to the image, so if you really want all the detail you can muster, a longer lens is liable to be the key. I would not, however, recommend looking at cheap mirror lenses for this (again, maximum aperture, and autofocus may be issues, and super-cheap ones can be quite soft). Gird your loins and expect a four-figure pricetag.
So, my recommendation would be to go for the longer lens. HOWEVER. 400mm lenses can be comfortably handheld. 500mm lenses, it's a matter of individual experience, strength, and handholding capabilities as to whether or not they can be handheld. And using a lens for the first time on a trip may not be the easiest way to gain said experience. I would highly highly recommend taking a whirl through Roger Cicala's lensrentals video on the Canon supertelephoto lenses, and his other youtube video on support systems for superteles, so you can get a sense of what's involved. This is a very far cry from using a consumer grade 70-300 (or even the 70-300L), in terms of the weight/bulk of the lens.
Personally, I'd recommend renting a 100-400L, or if you really have to go for one of the great whites, budgeting for an extra few days rental of a 500mm or longer lens so you can get used to lugging it about and handling it and possibly a gimbal head.
I also had to make this choice.
For the TC you must be 100% sure that it goes with your lens. The advantage of a TC is that it is cheaper, smaller, and lighter. A 150-500 will be more expensive, bigger, and heavier but a better aperture.
If possible I would recommend rental of a 150-500. This gives you the possibility to test it out for real. If you like it then buy it. It is an investment but if you like to take pictures of wildlife it will be worth the investment.
I'm not sure there's even a Canon TC that will work with that lens, which means a lower quality TC. And loss of at least one stop of light. All in all, perhaps not better quality than simply enlarging the critter when editing the photo. Not only that, but auto focus may suffer a bit, so that can degrade image quality as well.
Unfortunately, TC's work best on the better fast prime lenses, and are accordingly expensive.
Even buying a relatively more inexpensive long zoom has similar trade offs. There's just not a good way around the fact that you get what you pay for. Alternatives might be a good older used lens, or maybe even rental.
A teleconverter will magnify the image produced by your lens on the sensor, but will not produce a better resolution than your original lens had. It just cannot: it comes after the lens on the optical path. In other words, if two points were too small/far away to be distinguishable with your lens, the teleconverter will not allow you to distinguish them.
The teleconverter would be interesting the optical resolution of your lens were finer than the one of your sensor. This is usually not the case with modern cameras: a good lens will give you around 10 Mpixels of optical resolution (usually much less for a zoom, look for numbers on DXOmark), and your sensor is much finer.
So, you shouldn't expect from a teleconverter much better than what you get by cropping your images digitally to fake a bigger zoom.
The bad news is that zooms with focal length longer than 300 mm are usually quite expansive. But I guess that's the price to pay if you really want a telephoto ...
A converter will decrease your lenses lowest aperture, or make it less useful when it gets darker or shady. Plus, you are limited to a 1.4x converter, as the 2.0 converter will likely cause issues with the autofocus on your lens, since most Canon cameras require f5.6 or better to focus. The 2x converter adds 2 stops. At 300mm, your lens is already at f5.6, so adding one or two stops will cause it to stop autofocusing. I would avoid the converter route.
Therefore, getting a lens is the best option. Don't forget that you can rent a lens as well, which means you can likely rent a much better lens than you likely wish to purchase.