The problem with ashnod's altar is that usually an oath player will activate the orchard at your eot if possible (if he doesn't need the mana).
Then, you'd have to immediately sac the spirit, thus not being able to use the 2 mana.

You could also look at spawning pit, which also allows you to sac creatures, costs 2.
you can make a creature every second upkeep, then sac it to stack for example. Or you generate a lot of them at your opponents eot for an alpha strike. Or, as with altar, you just prevent having tokens, but until when? You somehow need creatures to win, don't you? Unless you can smokestack them out of the game.

I don't really know which one is better. Or if any of them for that matter. I would tend to the pit.

I would argue that Ashnod's Altar, in a MUD deck, is strictly better than Spawning Pit. The two mana generated from the Altar can be used to cast an additional lock piece, (i.e. Thorn, Sphere, etc.) while Spawning Pit does not give an immediate advantage. As a side note, the benefit from Ashnod's Altar only comes for MUD decks running Mishra's Factory, which, from my Vintage testing, proves extremely beneficial.

Both are defintely good cards, but lets look at the benefit of each. Ashnod's Altar lets you use mana NOW, while Spawning Pit could set you back a turn, potentially two, and potentially show no advantage because we will eventually sac the token to prevent an Oath trigger.

This advantage is why I argue that the Altar could be the superior answer to Oath for MUD. We can use the mana immediately, and we have no upkeep setbacks (i.e. sacrificing our upkeep for Smokestack and Tangle Wire to Eon Hub)