This is a pretty tough poll. Both physical guys, not especially offensively oriented.
I'm a Bruins fan, and I love what Adam McQuaid brings to the table. The dude is frankly scary as hell when he fights, and he seems like he fights with the intention of destroying other players. A mean dude. That being said, he's prone to some fairly serious defensive gaffes now and again.
I think in terms of overall value and play, it's a wash. If you're talking in terms of who plays defense better, I think you probably give the nod to Polak.
I value McQuaid very highly. I think hes on a tremendous contract and won't look out of place in the top four if hes needed there on a short term basis. Hes also much bigger than Polak, tough as nails and will fight almost anyone.
Better defensman? Talent wise I'd probably take Polak, but he also makes over a million more...and thats significant in your bottom defense. Would I trade McQuaid for Polak? Definitely not.