Next model up or go 27" or possibly 2011 27" model, or is the usb 3 that much better?
Also seen there's an insane wait time of 3-4 weeks for the higher end models....
USB 3 is a must have these days if you're going to be copying any large files or a large volume of files to external media. USB 3 is 2x faster in spindle applications (i.e. a 7200RPM connected to USB 2 vs USB3), and up 6x faster in SSD applications.
With that said, here are the geekbench scores broken down for all mac models so you can see the performance scores:
http://browser.primatelabs.com/mac-benchmarks
If you're happy with your current mac, the new imac is not a major upgrade, but if you are unhappy with things such as light reflecting on the glossy screen, the new imac has up to 75% reduction in glare. Or, if you're wanting to go USB 3 for copying lots of files to external drives, good upgrade.
Cons to new imac: 21 inch has no user serviceable ram, no optical drive, and no VESA mount compatibility. 27 inch one same cons but it does have user upgradable ram slot.