Which Saber should I use?

Probably. She's better than Gawain because she's a 5-star and her buster up isn't stage dependent. When her rank up rolls around, Seiba becomes a decent unit.

If you've got the mats, EXP cards and no other servant you'd rather raise then sure.

Otherwise Rama is the better boss killer and Gawain is a good enough wave clearer that until her instinct skill gets its strengthen unless you like her as a character I don't think whe's "worth" the investment if you already have Gawain to deal with waves.

Sure. Just make sure to NOT invest in her instinct.

From the gameplay point, you've covered both ST and AOE saber roles. Also Gawain is decently lvled but having another backup saber to cover you is also a good thing and she is an SSR.

I have both saber, Artoria is lvl 90 and Gawain lvl is 80 and they're doing their job well.

Bonus : put the uncle and the niece together and you'll see Excalibur and Excalibur Gallatine really often 🤣