Beautiful, wide open setting, great fight mechanics, interesting story, spot-on scaling from normal to vet to hm. And the giant specter of Hircine in the background is stunning. MoS wins easily over Fang Lair.

March of Sacrifices has its beautiful corners, and the general atmosphere is really nice for a while.
But only for a while. Then, the permanent twilight loses its spell, and you realise that wispmothers are pointless, lurchers are annoying, and chasing scattered enemies through dimly-lit woodlands is a bit less exciting than you thought.
I really don't like the Tarcyr fight. Enemies that force you to experience all their mechanics by vanishing at regular intervals, are questionable anyway, but having to deal with crouching all the time and evading random stuff, is not fun.
Bonus points for the "Baby Hircine" bug, though. That is cuuute.

Now, Fang Lair has boring, repetitive visuals and a colour scheme so grey that you wouldn't even notice if you were to play the dungeon on a black-and-white screen. Funnily enough, the most colourful thing in the dungeon is probably the large door in the boss room - a door that you cannot use.
That said, I think gameplay in FL is much better than MoS. I find the bosses more interesting (having a boss consist entirely of adds, the tiger called "let's teach interrupt", the ghost with chains, and so on - the lich boss can by annoying if he wants to). The mobs in between are - I don't know, more focused? And I like the mechanics on the last boss, except - again - for the colour scheme: blue-ish ghosts in a blue-grey-ish room with grey-ish enemies is like playing Tetris blindfolded, and the main reason why I don't have much interest in ever doing the hard mode again. At least they learnt a bit when they recycled the idea for DoMalatar.
But then, FL also has the nostalgia bonus of basically living in the dungeon for weeks when trying to get the Worm Cult personality.

And Fang Lair squeaks out a win by the skin of its teeth!