For me, there's so much random shit to do in SR2 that it's a blast to play. I haven't played Fable 2, but I can say that you can't dress in a Hot Dog outfit then use a cop as a human shield only to execute him after you're done killing people in Fable 2 like you can in SR2.
Buy SR2 for the laughs, buy Fable 2 for the simulation.
[quote name='Moxio']If you're paying $60 for either, you're doing it wrong.[/quote]
This too, without the B2G1 pricematch at BB, I wouldn't have SR2.