Unfortunately, there is quite a few CPU heavy synths and FX these days. The i7 is better for those, particularly ones like Diva that can multithread.
I cannot compare directly to the laptop CPU as I have a desktop (an OCed i7-2600K), but I *never* have the crackling options other people have with Diva. (Alchemy, which does not have multithreading, will occasionally give me a crackling sound if its complex and a lot of notes).
I would say that you are fine for now with the i5, but modeling is becoming more complex; the i7 is more "future proof". If you can only afford an i5, however, you can do plenty. I would, in fact, prioritize an SSD over a bigger CPU.