Mike Wallace or drafted rookie
Mike was our #1 WR. A rookie isn't going to be able to replace that production (even in a down year by his standards). A drafted rookie would be our #4 WR on our depth chart behind Brown, Sanders, and Cotchery.
Rashard Mendenhall or Dwyer, Redmond or rookie
Dwyer and Redman can be retained for 1 more season with an RFA tender. However, a rookie RB should be drafted, because I doubt that either Dwyer or Redman will be with the team after 2013.
James Harrison or Jason Worilds
When healthy, Harrison is obviously the better player, but he's rarely been healthy and isn't getting any younger. Because we are so cap-strapped, there may be no choice but to keep the OLB with the 6-figure cap number over the OLB with the 8-figure cap number.
Troy Polamalu or Robert Golden
Is this really a question? Golden is an 2nd year UDFA special teams player, not a starting safety. If we had the equivalent of a Worilds as a backup safety, then Troy might be worried about being a cap casualty. But with Will Allen and Ryan Mundy free agents, and only Golden and Cromartie-Smith behind Troy and Clark, this is the most dire position on our team depth-wise (we may draft two safeties). You don't cut Hall-of-Fame caliber starters at positions where your backup situation is a huge liability. You just don't.
Ike Taylor or Lewis (this is probably the only one where the existing option is okay - Lewis)
I'd love to have both of them back, but if it takes sacrificing Taylor in order to retain Lewis, it's a move you have to make. You don't let go of players entering their prime in order to keep guys who are well past their prime. You just don't. Without Ike, I'm not super concerned about Lewis and Allen starting (although I am concerned about the depth behind them).
Kiesel or Heyward/Hood (Kiesel outplayed them both last year)
Keisel's a good guy to have in our DL rotation (and I think we'd only save a few million if we cut him). I'd prefer to keep him for one more year, but if we need those precious cap dollars elsewhere, then there are worse things in the world than having a couple of former 1st round picks starting at DE in our 3-4.