Don't have Emiya Alter and never used Tristan, but Saberlot is an absolute crit beast and widely considered the best 4* Saber, if not the best 4* in general.
As for grails, there's not much point in grailing for pure practicality. The benefits are generally very minor. It only makes sense to grail if you're going to be using that servant an absolute fuckton and you really want every bit of bonus damage on them, or if you just really like them. Sometimes you get lucky and you get both. That said there was 1 case where I grailed for practicality first and liking a servant second, it was Summer Mordred to 90. Not that I don't like her, but that alone wouldn't have gotten me to grail her as I'm very stingy. But it just so happened that with the setup I had with her for farming QP doors, sometimes she'd roll low damage and not get enough overkill hits to fully recharge her NP, and when I grailed her to 90 she had enough damage to ensure that she'd go back up to 100%, 100% of the time. Aside from her, I have Kiyolancer and Tamacat at 90, and Illya, Sitonai, and Abigail at 100 and 2000/2000, all primarily grailed for waifu reasons. Illya isn't very good outside of bursting down big assassins with a Merlin or two, Sitonai is pretty good in the usual situations where an Alter-Ego is good, and Abigail is far and away my most used servant because almost nothing counters her and she brings utility and damage to just about any team.