( "My T1i is obviously not outdated, but I feel like I'm banging up against some of its limitations after a year of ownership.") I think that feeling is a good indication it is time for the upgrade.

You might have to define "recently". Many here have made the move up from the rebel to a 7D or 5D II. I bought my first 5D II almost three years ago moving up from a 50D (at that time there was no 7D). I noticed a big jump in IQ and camera ability.

If it is low light performance you want, go with the 5D II. It will give you the best IQ and AF and yes you will notice a difference.

I had the 7D for a while, the diffrence in AF between the two is mainly between static and moving objects. In my experience shooting static objects the 5D II performed as well, and sometimes better than the 7D did. The 7D did better with moving objects.

Both bodies will help you if you are having AF problems, you can do an AF adjustment with both the 5D II and 7D.

To me it would be a money issue that would decide which way to go. My first choice would be the 5D II and buy it knowing I would be selling my EF-S 10-22 to fund buying a new 16-35L II. If funds wouldn't permit that upgrade the 7D would be the choice.