The T6i would be an upgrade with better color depth, dynamic range, and ISO performance. But the SL2 is better at the same over the T6i.
But it may be better for you to wait or spend more now and buy a 80D. That is again better over the SL2 not just in sensor performance but also body, shutter, AF, and more. Right now 80D has $200 off for holiday sales. $1,000. Going from T3 to 80D definate noticeable upgrade! But sensor performance wise the SL2 is the best deal for upgrade.
So you're saying the SL2 is an even better camera than the T6i? I had previously considered the SL2, but I had leaned towards the T6i...I can't recall why now..
I'm thinking the 80D is out of the question for me at this time. I have no doubt that it would be a great upgrade, but I'm not looking to spend that much more with swapping out my T3. There has to be a cutoff somewhere! Otherwise the "well if you spend just this much more, you can get this" game would never end.