Marie or Medusa?

Personally I'd say Medusa just 'cause I don't feel marie has enough kick to her to make her survivability worthwhile. granted I have neglected both but medusa atleast has an NP charge skill.

Personally, I'd say Medusa, but that might just be because her waifu stat is EX. Plus she's done alright by me, being my main Rider class servant for most of the game so far. I honestly can't really speak for Marie because I don't have her.

Medusa is a great farming servant, but if you don't have many of the top-tier stall servants (Jeanne, Tamamo, Waver etc.) then you might consider leveling Marie. While her kit has a lot of problems (tankiness with no taunt, high star absorb with low damage, minimal ally support), there are a lot of missions in the future that require stalling through long fights (there's one out on JP right now). If you don't have those top tier stalling servants, taking a full 3-hit evade/invul team (marie, cu, cursed arm hassan etc.) is a great budget option.

But in the end both would be fine to level, and don't stress too much either way cus you'll probably max them both eventually anyways!

If you have strong backliner anchor on your team like Og Chu, Hercules or OG Nero.
Marie is a decent alternative for last man standing servant. Even you have her at NP 2 so she'll hit harder.
Medusa have good kit for killing bosses and farm purposes. Class advantage wise she and Marie will survive longer compared to those berserker.

Full berserker team is really fun to use, but it's really not recommended. Sometimes i use Hercules, Ibaraki, tamamo cat, lu Bu or Raikou and other berserker on the same team. Berserker life choose you, even if you don't choose them.

Also i prefer to raise supp servant first if i have strong DPS already. Though it's up to you.