Need quick advice for which lens to buy!

Hello, 
I am very new to the DSLR world - I recently purchased a Canon Rebel T3. I have a Tamron 70-300 VC lens that I'm in LOVE with but I'm now looking to upgrade my smaller 18-55mm kit lens. I take a lot of portrait shots (for fun but would like to do professional someday) but I really focus on taking images at my brothers' sporting events. They play football, basketball, baseball and powerlifting (an indoor gymnasium sport where they bench, squat and deadlift.) The issue is that my 18-55mm kit lens doesn't cut it for the close-up shots that I'm able to get when watching powerlifting or basketball. I love my 70-300 but sometimes it puts me TOO close, which is also irritating. I want something with better picture quality that will be able to capture the sports images as well as birthday parties, portraits, etc. 
I've received two lens suggestions for my broke, college-student budget.
First, the Tamron 17-50 non-VC  f/2.8lens (roughly $475 in a Canon store close to me).
Second, the Sigma 17-70 f/2.8-4 (roughly $369 on Amazon right now). 
Here's my thing: I've looked and looked online and I honestly can't decide. I've seen good reviews about each, but I'm looking for some professional help with my decision (which needs to be made this week). If there's other suggestions you'd have, that'd be greatly appreciated as well. I can't decide if I want the 17-50 with 2.8 all the way through.... or if I'd rather have an extra 20mm of zoom and lose te 2.8. I honestly know LITTLE about everything else, I really just enjoy using my camera for the little things right now. I'm looking for a lens that will last and be of good use for me. 
Again, any suggestions are appreciated!