Everyone who plays SRTT seems to either hate it as a terrible alternative to GTA or praise it as so much better than GTA. No exceptions.
I thought it was an enjoyable game, and different enough from the seriousness of GTA4 that I don't really look at one as better than the other. Overall, I enjoyed both games, and enjoy both franchises. I'd probably prefer that they stay different enough from each other so that one doesn't truly feel like a rehash of the other.
If I had to pick one to buy for PC, it'd be SR3, since from what I understand, GTA4 still has issues on PC.