The TCR is a little stiffer, twitchier on descent, more responsive. I always felt that the Tarmac (at least the '06 comp and expert frames I test rode) was trying to do too many things well, and ended up being just a bit compromised on all.

BTW, I ended up on a '06 Allez Expert; it's stiffer than the lower end Tarmacs, not as twitchy as the TCR, and ascends well, even though I pretty much suck at climbing. Of course, it's not quite as comfy as the Tarmacs, either.