I have the 3.1 GHz i7 with Fusion Drive and 16GB RAM. My wife generally uses CS6 apps like Photoshop and Premiere Pro along with Chrome, Safari, etc. In the first few weeks, I have been checking activity monitor off and on. Based on this, I would rank the 21.5 "options" in order of most important to least important:
1. Fusion drive provides definite improvements in performance, opening files, etc. Having a slow hard drive would be noticeable for sure.
2. Looking at the system monitor, I've been over 8GB of RAM a few times. But, most times, there is less than 8GB used, even with 6-10 apps running at once. If I only had 8GB, the iMac would swap to the fusion drive with minimal loss of performance. So, that's why I feel that the Fusion drive is more important than the extra RAM.
3. I rarely see on Activity Monitor that the processor uses much more than 25% of its capacity, no matter what I do. So, I'm interpreting that I have much more capacity than I need today. Doing SD video on Premiere Pro is super fast. And, I tried to make the processor "work" by doing different Photoshop effects. Every digital effect that I tried was pretty much instantaneous.
So, thus far, I'm not seeing situations where the processor appears to limit performance. I expect, for what we do, we have more machine than needed. (Of course, if I had a slower machine, it's possible that I'd have a different opinion.) Down the road, I plan to do HD video editing, which I know will be much more challenging.
So, if you want to have no worries that you bought too slow of a machine, spend the money for the upgrade. But, if you're doing stuff like I described above, the i5 processor may be just fine.