I was in the same boat. Although I have not bought one yet, I'm saving. I'm going with the 24-70. Big deal with IS, its nice to have, but on a lens that doesnt go over 70, its not really needed. Just dont shoot under a 1/80th @ 70mm to be safe, however that 2.8 will be better then the IS for two reasons.
1. IS works for 1.5-2 stops. f/4->f/2.8 is only one full stop, so you technically lose a but there but 2.8 does bring in a lot of light.
2. 2.8=fun. At such a small aperture, there is not a lot of depth of field, the range is quite small, which can be great for a lot of things and fun to mess around with.
granted the 105 is a bit cheaper, typical you only need to buy a lens once. go with the more expensive one.
Btw, what camera are you using?
Hope this helped and good luck.