I bought a T3i last year (has been only just over a year?), not knowing how seriously I would take this hobby. I ended up collecting a set of good EF-S lenses for it.
I also bought a bunch of good EF lenses (e.g. 24L II) that are great on the T3i. And the 100-400 is fantastic for birding/wildlife on the T3i.
But I wanted to experiment with the shallow DoF and different effective FLs I could get on a FF body. So I recently picked up a 5DII. And with the 24L I get fantastic DoF control, a wider FoV, and better ISO performance. I will not get rid of the T3i: birding with 100-400, remember. And the EF-S glass coupled with the T3i makes for a much lighter travel set-up.
It's the same reason I have both a Gibson Les Paul and a Fender Stratocaster: the nature of each "toy/tool" lends itself towards certain styles and modes of use. Yes, you can get by with one or the other, but having both is quite nice.
YMMV, and clearly if you are going to have a limited budget for a long period I would stick with the one that gets you the most bang-for-the-buck (I would suggest APS-C with a bunch of good EF-S lenses and whatever EF lenses you "need" to cover the FLs/f-ratios you desire).
IMHO, YMMV,
shinksma
