hmm i'm personally not a apple person, but that sounds like a nice swap.
I would suggest though that you do as nihil suggested, get the deal in writing with witnesses, and maybe do a 2week temp swap, just so you both can test the computers at your own leasure. Then if both are happy then deal is done.
But only do the deal after the trial period if you are both happy with the trade, do not swap if you are not happy just to please your friend.
Make sure that it is in the contract that it can only be considered a deal if both party's agree after the trial period that they are happy with the agreed swap. And that there are no refunds.!

cheers
kris.!