All things being equal, how much performance difference would there be between the two Shimano wheels, and between the RS81's and the Mavic's?

I'm looking for a set of wheels - budget aero-ish wheels if thats not a contradiction in terms.

Other possibilities are Planet X 50's or cx 45's, but for my purposes, they're exactly the same price as the Shimano RS81's but I've read that there have been hub problems with them?

Can anyone shed any light on these wheel sets?

As another aside - which tubular's would come closest to budget aero or be closest in price to the RS81's?

thanks

andrew