I'd take McClain over Cambell in this situation. Most likely we're going to be drafting mid to early next season too. Why not get the best LB in the draft this year, and hope for better than the 4th best LT in next year's draft? How much success is a rookie LT going to have with on this team in year 1 anyway? He's going to be in the trenches next to a second year player. I'd rather address LT through Free Agency or via a trade (any takers for Marshawn?).
Bringing in McClain means giving a rookie a chance to succeed. Paired on the inside with Poz on a 3-4, or moving Poz to the outside on a 4-3, he'll have a chance to become an immediate impact on defense. Sometimes you have to put your best people in your biggest oppurtunities, rather than your biggest problem spots.