I had the exact same spec computer as you when I upgraded to the new imac i7 (except mine was a mere 2.16 not a 21.6
).
I noticed incredible performance leaps using Ableton with rewire to Logic. I know that the new cpu obviously made a difference but being able to load more ram than the max 4gb in the old computer made equally more of a difference imo. I went to newegg.com and got some ram to upgrade it to 16gig of ram for around $100.00. When running several plugins on certain tracks (using Native instrument especially) on the 2.16 computer I frequently would see my ram making the computer come to a near stand still. I looked around the web before I made my imac purchase and most posts from gearslutz, the apple forum discussions, ect. made it pretty clear that ram was the main thing that would improve performance which seems to be true. I personally ran into more error messages concerning ram on my old comp than any kind of cpu overload messages. There is no doubt, since you are coming from the same boat as I, that you would see SIGNIFICANT increase performance by just using the 15 with more ram if price was a factor in getting an i5 vs. an i7. I bought an i7 merely because I wanted to kind of "future proof" the computer since I keep my computers for 4-5 years before I upgrade.