Scathach or Ozymandias?

Hands down Scathach, unless you seriously need Ozy for fan/love reasons. Setting aside utility, who is better, whatever, Scathach is RARE. she will come out a handful of times in a year. Ozy as of camelot's release will always be available and will spook you eventually if you play enough/roll enough. Scathach is only available when she is. If you seriously only care about power/the stronger of the two, honestly its comparing two different things. do you compare a track runner with a boxer? at that point, its really what your playstyle is and what you favor team composition wise, but I think skipping scathach for a servant always in the summoning pool is just silly IMO.

I would go for Scathach also : a premium boss killer (except against sabers) with a guaranteed stun on her NP, 2 skills that buffs her damage to an absurd amount and an Evade skill. Her NP can do more than 300k against Divine Archer (like Gilgamesh and plenty of servants in the Archer class).

Her only drawback is her NP gain: very inconsistent but overall, a great servant.

I never played the JP so I don't know about Ozymandias but when I look at his profile/skills/... he is a very interesting servant with Charisma, Imperial Privilege like Nero, a way to increase the success buff (or make it 100%) on some skills that don't always activate (Like Nero's second Skill or Scathach's first skill), a party charging NP Skill and a ST NP.

The "problem" is that he need a lot of new materials available only in Camelot to ascend him so you may have to wait till an event to fully ascend him and he will be always in the summoning pool (like Spooky Vlad). Scathach is limited and you may get/have all the materials to fully ascend her.

Current content, go for Scathach because she's good and deals absurd damage with her NP (no multiple HP bar yet). For future investment, Ozy by far because he's arguably the best Rider in JP.

Ozy for me
Scath's NP gain hurts, and she is a dedicated DPS, she has no other utilities beside that. For lancer DPS, I can always use Cu or Inshun, both literally doesn't need setups and can solo every archer bosses you can think of.

Ozy is an utility rider, aside from damage, he can also support. His NP does devastating damage, has nasty NP seal effect and plenty other debuffs. Being a rider, he is a star hoarder, perfect as crit DPS. His skillset benefits both himself and party, can be a decent support. Even if you put him into a party with another DPS, he can still works.

In the game current state, Scath is better because burst damage is what you need, but later, definitely Ozy, he is more consistent.

Don't mind about being limited or whatnot, if you don't need a limited servant, why bother rolling ? That's just me though

I won't bother with Scathach cause she is after Raikou and once Memelin comes out all Quick servants will be a step behind in the meta cause of lack of Quick support.