I haven't seen any of either series and I've heard a lotta junk about both series and also some praise (not that much) but the important question: which is better? Or if you think neither are good; Which is least bad?
Enterprise by a very long way. It had two excellent seasons and several good episodes in the bad ones. TAS, as much as I love it, has one good episode.
Enterprise by a wide margin. The only essential episode of Star Trek (Animated) is D.C. Fontana's "Yesteryear."
I would say Star Trek Enterprise is better by a wide margin. TAS was for kids in the 1970s. It can be daunting at times. Unless you are like me and remember it as a kid, and want to see it out of nostalgia, I doubt you would enjoy it much.
I don't think there's any comparison. Enterprise is much better than TAS, IMHO. Particularly, seasons 3 and 4 of Enterprise are really good.
TAS has got nostalgia value, but realistically a half-hour series (really 20-something minutes minus commercials) made in the 1970s will never be able to compete with something that has hour-long episodes and live-action actors and was made in the 2000s. Even if it fails compared to some of the other live-action Treks, ENT has got more scope and sophistication than TAS could ever manage with its format. TAS biggest failing is that you just can't do the same kind of character studies in the half-hour animated format as you can in live-action, so by necessity most of TAS's scripts were geared towards story narrative and nothing else. Yesteryear was the only exception to that rule.
Heck, I'd even say seasons 1-2 of Enterprise are more watchable than TAS. ENT had more decent episodes that are watchable, even if the facepalmingly bad ones cropped up every other week... as opposed to most weeks in TAS.
ENT by far... Not hating on TAS though. I just think it's more of an ST curiosity than something that can put next to the series and movies.