Linney as Regina and Nixon as Birdie would be my vote. While I've heard great things about both pairings, that particular combo is a) the Tony nominated roles and b) I personally prefer the "on the verge", nervous Birdie that Nixon plays and the strong, sharp Regina that Linney does. The reverse (from clips, I haven't seen so I'm not the best judge) doesn't fully embody how I personally envision the roles. Most (if not all) critics saw and raved about Linney's Regina/Nixon's Birdie too I believe. I think either way you won't go wrong at all though.