ipad 2 is faster, slimmer and you can get the magnetic flappy front cover for it.
ipad 3 is basically an ipad 2 with a snazzier display which 99% of the time you won't see the difference. Not unless you REALLY look for it. It's also heavier as a result of the bigger battery they had to shoehorn in to power the screen. Oh and also as a result it takes much longer to charge.
Other tablets....the Nexus 7 seems to get a lot of publicity but a friend of mine was saying today he returned his because he found it sluggish.
The only other non-ipad I'd personally consider is the Transformer Prime but given the price I'd probably just get an ipad.
Rumour has it though that Apple are releasing an ipad mini soon. No idea if it's true though.