The thing is -- and this is coming from someone who is on Mini #3 (had the first C2D and the original G4) -- the Mini will NEVER compare to the iMac in terms of performance.
The main appeal of the 27" iMac is going to be if you want the highest-end hardware Apple can give you. If you do, there's no question left to ask. If you don't want the highest-end hardware, then it's a different story
Is there a ease of upgrade? Yes, that's hard to argue. However odds are you're going to get more mileage out of the iMac between replacements. Upgrades are more expensive, but also further apart (unless you want to always have the best you can get). Mac Minis are always analogous to a MacBook Pro 13. iMac 27s always offer the highest-end non-workstation hardware Apple offers. The performance delta between a max specced Mini and a max-specced iMac is always greater than 100% (CPU performance, GPU performance, memory capacity, internal storage capacity/performance).
Right now a 27" iMac can give you 300% better CPU performance, 300% more memory (100% more if you don't want to spend $3K on 32GB), 300-400% better GPU performance and at least 100% more disk space (and a considerably quicker configuration) than a 2.66GHz non-server Mini... if you go for the top-end SKU. Not cheap, but the option is there. It's never there with the Mini.