If you could really trade for one of the top 5 players in the League, a LEGIT MVP candidate at one of the most difficult positions to fill, you do that without question. In the proverbial cocaine heartbeat. As always, a lot will depend on the perceived likelihood of getting Rondo to sign an extension this season that's not financially crippling. As much as I love Ray Allen, 2-guard *is* probably the easiest position to replace; with a healthy KG, refreshed Pierce, and CP3, we'd need an Ariza-type at the 2-guard, someone who can take some of the defensive pressure off PP. Those guys, you can find.
Love Ray Allen, love Rajon Rondo, really, I do... but CP3's the best player in that deal, by a good amount. You make that deal.