Hi Dave, sorry to say that you can't do either...
In order to "crossgrade" from Mac to PC (or vice versa), you must be on (or upgrade to) the latest version to do it... That's because Adobe doesn't sell, stock, or offer any older releases for either platform.
And on your proposed alternative regarding the academic versions - because of the heavy discounting, the education editions have the limitation of being non-transferrable.
Sorry they're not the answers you were hoping for!
But if you decide to reconsider your stance on upgrading, note that Adobe is currently offering a grace period for free upgrades to CS6 if you purchase CS5.5 now.
Anyway, hope this helps clear things up