Grailing an Alter

"Grail for love" is the way to go. If you love Lalter and plan to use her, then go for it.
Congratulations on your yolo roll.

if you're really planning to grail based on utility go grail cu boi instead. Great lancer, even better grailed. U don't need to grail a wave clearer unless its out of love. well if you must, grail Lalter as more attack will make her crit harder. Latoria is a wave clear and won't need those extra stats.

i grailed all the baber alters (salter, lalter, santa) outta love, but i'll give you an answer based on gameplay.

the differences between a 90 lalter and and a 90 lanturia is that lalter's np hits harder, has a higher mana burst, and is gets that crit damage. lanturia can farm more efficient due to that np skill. depending on your style is how you should decide.

personally i think you should grail for love. if you love another servant out there, hold off on that grail. lalter can more than hold her own against most enemies at 80.