I agree with trentvaughn. My initial thought is that I wouldn't trade two top tier WR's for one top tier RB. If you can make a one-for-one trade and really need the RB then do it. The question is: How many RB's are you allowed to play and who else do you have at RB and WR?
Bottom line: if you are stacked at WR and really don't have a good RB, then do it. But if you are looking pretty good at both WR and RB, don't make the trade.