I think it's pretty simple. If you ever enjoy watching motorsport on TV, and enjoy the idea of getting closer to the reality of that kind of sport, then get the sim (SCC). If you just want to have fun, and don't mind falling into fantasy, dramatisation land, then Shift may be for you.
Overall I have to recommend SCC, by far, over Shift. SCC has better car handling (especially for wheel users it seems). SCC allows private lobbies, which is very important.
JJ mentioned SCC having a tight, but tiny online community. Well for me that's another reason to get SCC - You're guaranteed good, clean racing if you want to race with us, even though a few more numbers online would help. Plus, we've got competitions running on this forum which are very enjoyable.
SCC has updates too. There have been two so far, with one more coming soon. While I feel they've made a bit of a mess of the development, it's still the best choice on the PS3.
If SCC got a push in the market and proper distribution, I think you would find maybe half of the sim users saying GT is better, and half saying SCC is better.