My head's big enough without thinking my opinion is that of an expert.
Shelton is younger and I believe less talented. KFFL reports the Cardinals are asking as high as a second-round draft choice, which I certainly wouldn't pay. He struggled with back and knee problems this season, after missing some time in 2003 with other injuries.
Miller is 32 years old, and while he's not exactly the spitting image of health either, if the Titans cut him and we can sign him for a low to moderate contract, I'd prefer that over trading for Shelton. Miller is a pretty good run blocker, and he's gotten better with age. Plus, he's familiar with our system.
As for other options out there, someone like Jets UFA Kareem McKenzie of or RFA Brandon Gorin of the Patriots could be adequate signings. It should be noted that McKenzie is represented by Kevin Poston, and Gorin would only cost seventh round compensation, I believe. If we're going to give up a second round choice for someone, I'd rather it be Oakland RFA Langston Walker than Shelton.