Welcome to the Canon crop challenge, Frumper15!
My last camera was a 60D- essentially the 'semi-pro' equivalent to your T3i. Same sensor, articulating screen, though the 60D may have had a faster AF setup and more sophisticated AF- but it has that over my 6D too
So, here's what I think:
50/1.8 II- build quality is nary the issue for an amateur, but results are. Here, this lens fails at background rendering due to having an aperture with five straight blades. Sure, your subjects will be isolated and your backgrounds blurred, but they will be busy and point light sources will be rather ugly pentagons. Probably worse is that the AF is neither particularly fast nor extremely accurate, and you will have to be persistent to get sharp results. The better points of this lens are that the lens is cheap, it's light, and on your T3i, it's as sharp as any 50mm auto-focusing lens available for the Canon mount.
50/1.4 USM- better build quality to a point, and faster AF to a point, but on your T3i, the main benefits of the upgrade are the more sophisticated aperture resulting in better background rendering and the capability of full-time manual focus override, and if you're a polarizer user, a non-rotating front element. This is the 50mm lens I use with my 6D, but I must mention that it's IQ advantages and faster AF are largely limited to newer FF bodies. I can only really recommend it for a T3i user if you must have 50mm and are serious about the quality of your photographs.
Other 50mm lenses for Canon- there are plenty, but none that stand out as terribly useful. The Sigma is a basket case for AF, the high-end Canon 50/1.2L is slower focusing and not sharper than either more accessible Canon lens above, though it does have wonderful rendering, and the rest are expensive (ranging to extremely expensive) manual focus designs. The harsh reality is that Canon desperately needs to introduce a truly 'great' 50mm lens for their mount; there is a rumored 50/1.8 IS USM in the works, similar to the family of wide-angle primes they've already introduced, that would likely solve this problem. No ETA, unfortunately.
1. 40/2.8 STM for indoor shooting. Wider than 50mm, which will be constricting on your T3i, and also with a much wider rounded aperture than your kit lens and extremely sharp wide open with accurate AF. Truly a gem of a lens on any camera to which it's mounted.
2. 85/1.8 USM for actual subject isolation. Better build quality than the 50/1.4, same price, with lightning fast AF that hits it's targets. This was my second favorite lens on my 60D, though it will be too long indoors for much more than single portraits.
3. 35/2 IS USM for real indoor work. Expensive, though B&H is letting them go for $500 at checkout, and worth every penny. I used the 24/2.8 IS USM, my favorite lens on my 60D, which is of the same family. Fast, accurate AF, the best background rendering at 35mm on Canon, and sharper than the aging 35/1.4L and almost as sharp as Sigma's new 35/1.4 Art, both of which are significantly more expensive. If you could stretch for any lens, this would be it. The non-IS version that you mention above isn't really a great alternative, since it shares every weakness of the 50/1.8, though like the 50/1.8, it is definitely sharp in the center.