Jalter or Merlin

You actually got a very good servant in Sanzang, as for the damage output thing, it is kinda true, due to Casters getting fucked by their .9 class attack modifier, but she is definitely worth investing in, mainly because she can spam her NP like crazy, she has a pretty good damage cut on her taunt, she ups the parties NP gain and Crit star drops, and she wont die in 1-4 hits to absolutely everything like most zerkers.

Merlin is arguably the best servant in the game and it will be worth it to pick him up, Jalter is good, but is better with Merlin around, and double Merlin comps are nearly brain dead in how busted they are.

Merlin is a broken support used in almost any comp, and Jalter hits like a truck with or without any supports. None of them are necessary to beat the game, so I suggest you roll for what YOU want more.
I'd personally roll for Merlin because I want to deal tons of damage and he works well with a lot of servants.

Same, a F2p as well. I got Nightingale which sucks, I wish I got Waver or Tamamo. I'm might as well roll for Merlin cause I don't have a support caster as 5 or 4 star.

Merlin works well with everyone and is broken in buster crit. Considering you have mom you would be future proofing your team if you can roll memelin. Jalter actually requires help and cannot clear harder challenge quests without double waver or ironically enough merlin lol. Xuanzang actually fills the niche of being an assassin killer and some quests are really hard without class advantage and if you have merlin she is a buster servant. Zerks can get blown up with something unlucky like 3 crits in a row(Praise RNGESUS). The moral of the story is roll for memelin, watch the FP roll in? buster memes and quick is dead

Merlin.

For Merlin or Jalter just pick whichever one you want more and unload all your SQ on one of them. Everyone on your friend list will have one of them so having both isn't really necessary. Now about Waver, as long as we get the same treatment as JP, there will be a paid gacha that is broken up by class. The caster gacha will only have Waver, Sanzang and Tamamo as possible 5 stars. I understand if you feel burned on this gacha and don't want to try at another one or the 33% chance of NP2 Sanzang isn't worth it, but a Waver or a Tamamo would open up your team building options quite a bit. Just something to think about.

It is safer to save for Merlin since he's an absolute broken support who works with basically anyone. Jalter on the other hand is a crit beast that deals massive single target damage to almost anyone due to the Avenger class. However, the latter does need some setup and specific team comps, restricting her use in certain levels and challenges.

If you can't go for both, Go for Merlin. Ruler Enemies are very rare to find in the game and most aren't entirely terrifying per-say besides Martha and Amakusa who are the only offensive. JAlter is a Buster Goddess of the whole game, But she will only shine against enemies that are Berserkers or Rulers other than that she will do impressive but not too high of Flat Damage. To utilize Jalter's potentials she needs attack boosts and buster boosts to do impressive Nuke damage, Add in some C Stars then she will melt the game. Merlin is different, He is the Go To Caster for all Utility, From Buster Boosts to NP Gains and Impressive Healing. You won't lose a single servant on a Merlin/Double Merlin team. Xuanzang is perfect on a Merlin team since she has amazing NP Gain and high damage for a Caster. While both are amazing in their own right, Jalter can be replaced with Herc or Cu Alter if you want consistent damage 24/7. Merlin is basically Waver just more OP about it.