Who should I grail

I think Lancelot is better but eventually they are used for different things, so it depends on what you need more or who you like most. SAlter is a faming servant, while Saberlot is ST boss killer.

If you're not grailing your waifus, Grail saber Lancelot. He becomes on par with the best SSR sabers at 90 due his monstrosity of a crit playstyle and hard hitting np with defense down

Putting the grail you waifu thing aside
Salter is basically used for farming or to clear waves due to her high np damage so she won't benifit from the extra stats
On the otherhand Lancelot will be used during boss fights and if you don't have an ssr st saber ( currently only okita ) then he is the better choice to grail
Also you can wait for musashi that has a buster st np so she works well with merlin
Conclusion you can grail lancelot if your not rolling musashi and don't have any other st saber or save the grails for other servants

For gameplay? Lancelot, he's a crit monster and being ST makes him awesome as a damage dealer on hard fights, but the answer to this question will always be the same, grail whoever you like the most.