I think I know what you're saying.
The imac will be a bit better than the mini, but not by a huge amount, probably. You will get a benefit because (I think) the graphics card in the imac is not integrated with the main memory. So, that's great for gaming and heavy editing, etc, but for usual tasks I don't know if you'd notice the difference.
But...I think all of the c2ds aside form the mini have wireless-n, which is AWESOME. I got lucky and it came on my macbook without me knowing, so I bought the AEBS and never looked back. So damn fast. For wireless...
Also, seeing as how you presumably have a dvi monitor for the mini, you can double your screen real estate. That, for me, would be the most compelling point.
But, don't think that upgrading the imac will be easier than the mac mini. If you mean changing the ram, then who cares? How often do you do that, even if it is difficult? If you mean anything else, i.e. hard drive, processor, video card, whatever, you can't. Not without voiding the warranty, of course, and certainly not without knowing exactly what you're doing. If you want to drop in upgrades, your only option is the mac pro. But...not exactly the same price range.
Hope that helped, i just ordered an imac myself.