Got a healthy amount of Saint Quartz, should I save?

Whomever you love more.

From a pure gameplay perspective though, Merlin.

Depends. If Jalter is ur waifu, spend every single quartz you have on her. If not, then go for Merlin because Jalter is just a buster crit servant. She's the best buster crit servant, but there are others who can benefit just as much from Merlin. Merlin breaks the game

You want Jalter for waifu roll for her, but if you care for gameplay more than everything else save more and roll for Cheat Engine Memerlin.
I have to give more warning, this is based by reality of countless master from Chaldea.
In FGO there's 99% Salt and 1% chance to get your favorite or desired servant.
It's like Romeo and Juliet. Even if you got 1000 SQ, there's no guaranteed chance to get that servant. It just increase the roll chance to get him/her.

No offense but i just tell you the truth, I hope you're lucky and get what your heart desired : )

It also looks like we get another chance at Merlin next year anyway but next rate up for alter is predicted to be in 2020 so take that as u will

Is jalter your waifu? If yes roll her, don't if not
Is memerlin your husbando? Same as above

If you love them both equally then i would say roll for meme because of gameplay (and he can grow d**ks on everything and everyone)

As for now i'm saving for meme and i'm gonna roll him with all the 730 quartzs i will have saved by the time he comes around