Would you like another point of view?
From the list of programs you've provided I guess you are getting the computer for home usage (iTunes, iPhoto/Aperture, watching videos, browsing the Internet and so on). Neither of those programs require fast processing speeds.
Speaking of raw processor + RAM speeds, here are the results both machines you're interested in scored: (source:
http://www.primatelabs.ca/geekbench/mac-benchmarks/ )
iMac (21.5-inch Mid 2010) Intel Core i3 540 3.07 GHz (2 cores) -
5670
iMac (Late 2009) Intel Core 2 Duo E7600 3.06 GHz (2 cores) -
4210
(32 bit performance, higher score is better)
The i3 appears to be ~35% faster than C2D. On the other hand, here is the score of the base iMac 21.5-inch 2011:
iMac (21.5-inch Mid 2011) Intel Core i5-2400S 2.5 GHz (4 cores) -
7257
And the base 2011 Sandy Bridge appears to be ~27% faster than the i3 and ~72% faster than the C2D. It is safe to assume that the 2012 base iMac Ivy Bridge will be ~20% faster than 2011 base Sandy Bridge (and twice faster than C2D) and so on.
I think either of the computers you're offered would do. If you're looking into exporting 1080p videos even the i3 will be ridiculously slow (it took it 30 minutes to export a 3 minute slideshow in Aperture). Where you'll notice C2D is slower is when you import your CD library or process very large RAW files or heavy batch jobs in Aperture. However, my mates run Photoshop on even slower C2D machines - one of them is a semi-professional photographer. Both computers can run Lion OS so they are future-proofed
The only question is how much you'd like to spend on a computer. Had I been in your shoes (and my present knowledge of Macs) I'd have saved some money and got the C2D.