That's an easy question. Ray is getting older. Even though it's not fair to judge him totally based on his performance this post-season, where he had to be the 1st or 2nd option on offense when he works best as a third (getting open looks), he's probably only going to go downhill from here. As an expiring contract, he's never going to be worth more than he is now, and we could maybe get something decent (younger, more athletic, bench guys we need etc) for him.
I don't believe we can afford to trade Rondo unless he is truly a huge problem in the locker room. His potential is just too great and I don't think we'll get fair value for him.
Don't get me wrong, I'm not thrilled about trading anyone in our starting lineup, but if I had to trade one of them, it would definitely be Ray. The other guys are just too important.