Hordes of the Underdark vs. Mask of the Betrayer

I'm probably in the minority, but I liked HOTU more.

Not because of superior story, but better companions. The only Mask of the Betrayer character I really liked was Kaelyn the Dove.

Wow, really? MotB was the first game I ever played where I actually enjoyed all of the companions. Safiya, Okku, Gann... even One of Many. I didn't expect to like Kaelyn because I'm a huuuge Kelemvorite and back in the day I was always rolling clerics for him, but she surprised me and I really enjoyed her character. Maybe not as much as Safiya or Gann, but still.

edit: It helps that most of them were some flavor of Neutral. I like my companions on the morally ambiguous side.

The Underdark is my favorite D&D setting ever, therefore I would choose HoTU over a lot of games, not only MoTB.

Sentimentality will make me choose HotU every time, though the gap between the two is razor thin. The OP specifically asks which one did epic levels better, and, again, I have to give the nod to HotU. I think Hordes does a better job of meeting the challenge level of my character from the point I've hit the epic levels. I'm currently playing a monk with a couple paladin levels and I'm pretty much stomping my way through with little resistance, whereas showdowns with Maugrim, Maker's Island, Vixthra, the fight before the crazy wizard guy whose name escapes me are all still challenging even after over a decade's worth of replays.
This said, I'd say any melee-first class is overpowered in Mask, especially if you have prepared yourself to deal with the fear effect of the Hag. The Spirit bar added to a spellcaster's need to rest adds a layer of complication.
In Hordes, I have to say that if any class is overpowered, it'd be a melee-first class that has taken levels in RDD.

Liked both.

Though I know what they were trying to do I did not like the Spirit Bar at all.