These are such pointless questions in a vacuum. The answer obviously depends on the players involved. Put it another way. Would you let JJ Watt go and sign several players for ~$5mil a piece? Re-signing Watt would mean several other positions would have to be filled by minimum type players whereas letting him go would mean you can fill several positions with good ones. In the end it's all about the overall potential of the picks involved along with their bust factor. Clowney, if he's truly the next Julius Peppers, I would not trade for 2 middling 1sts and a 2nd rounder.
Teams should draft best player available. Teams need to move up or down so that their picks are at need positions as necessary. Texans need a NT, they traded up to get Nix as he is more likely more valuable for them than 4-1 and 5-1. Even though Texans have been poor in the 3rd round, I love the idea of trading picks after the 3rd round to move up into it.