First off, make sure you do your homework before you get either of these lenses. The sensor size of the T3i is an APH-C sized sensor, and I believe both of these lenses are for full framed sensors. So as long as you are okay with the crop factor with these lenses, then we can proceed.
I just got a Tamron 18-50mm f2.8, and I feel that between your Sigma and my Tamron, they are very near the same, so when I say my Tamron is AWESOME…I’m sure your Sigma is great too.
I also have the exact Canon “Nifty Fifty” 50mm you’re talking about. It’s nice, but with that crop factor (I have a Canon 60D, which has the same sized sensor of the T3i), so it acts like around a 60-70mm lens. That kind of throw may be fine with you, but having no option other then backing up, I needed to get something wider, like my Tamron. Also, that f1.8 is nice for low light, but unless you’re a master at focusing (and I REALLY mean it…MASTER), your shots are going to be constantly out of focus. There may be like…2 awesome focus pullers out there that can keep a tack sharp focus at f1.8, and I’m betting you’re not one of them, lol.
My suggestion is get both! The plastic version of the nifty fifty is only around $100, so why not? But if it comes right down to it, get the Sigma, you won’t regret it. However, like I said before, do your homework and see what sized sensor the Sigma is for…and if it’s full framed, see if you’ll be fine with the crop factor.
Cheers!
Ryan~