Soggy, here's the info that I've gotten over the past year of research myself after being in the market for another shotgun. First I would probably lean towards a 12 ga verses a 20 ga. I know this is an arguable subject, but the gun manufacturer's and ammo companies have given me some of the ballistics and data on file, and with the wider variety of ammo selection, energy and performance I've personally seen in the field, the 12 makes more sense.
As far as your gun selections, what I've heard from people that have them and read myself. The deer slayer is supposed to be outstanding. It's got a screw in barrel like a rifle does, and accuracy is supposedly amazing. As far as the Savage 210 goes, they have had some major ejection issues, if you don't use super high rings your shell will not eject, and for the gun to function properly as per their design, from accuracy to ejection etc, you must use 3 inch slugs, and that came staight from savage. As I have spoke with multiple techs over there. However it is a very accurate gun. Again from the info I've gotten from the people that have shot this gun the H&R slugster is probably the most accurate of the bunch, it's a 10 ga bored to a 12ga so it's heavy and awkard but it seems that the one that everyone points to. I've had it in my hands, to funky and heavy for me, but it's priced right at about $250.
Curious to see what other feed back you get here because I'll be making my decision pretty quick myself. The only difference is I've got a chance at an 1187 brand new with a monte carlo stock, R3 pad, and fully rifled barrel for $680 that I have to decide on in 2 weeks. Hard to pass that up.