You might want to ask this in the McIntosh forum, but they're both good amps, it just depends if you need the extra power to drive your speakers.
I have the Mc2105, which is like the 2100 but with meters, and it's really more amp than I need to drive my Klipsch Epic CF-4's, but then again who didn't like a little extra power in their car? I'd drive a Cobra 427 instead of a VW if I could, but it wouldn't really be needed, but I do like the extra power!!
You like the looks of the 2100 better, then go for it, as you're the one who has to keep looking at it! If it needs any going over, may I suggest you send it to Terry DeWick, like I've done with my "new" Mc gear, and have him go over it; very reasonable work of the highest order, and a great guy to deal with. He also loves and owns Mc gear himself!
Best of luck with your decision either way, you can't go wrong with vintage Mc gear!