I'm normally one for telling people to not worry about waiting for the next upgrade, but it does seem a bad time to buy. If I were in bpsmiths shoes in Feb I'd have done the same for sure, even though an upgrade was
still being rumoured back then... but now the Ivy Bridge upgrade has hit the Macbook Pro and Air ranges, it seems logical the iMac and Mac Mini will follow soon.
You'll find the performance of an Ivy Bridge equipped model will be somewhere between the two models you've listed above, particularly in graphics performance. Either of those will do a great job, but unless you
need it now, waiting seems sensible.
Or, buy second hand or refurb (as above). Even if an upgrade does then come along soon after, you'll have paid less, so who cares