Silque gets the first strike and OHKO's Terrors and she usually doubles them. The only added defence Fay gets is availability and you can Grind in Echoes so it really doesn't matter, not to mention they both automatically learn it when promoted
No, she often doesn't ORKO them, at least not when she first gets it. Faye does though and once again, has more HP. Even if Silque had everything the same as Faye, Faye's HP advantage makes her the better physical tank anyway. Seraphim also does a lot of damage to human, and especially armoured units, but Silque definitely can't be thrown near them, but Faye can.
And the thing is, you can grind in Echoes, but I never grinded Faye. At all. When I reached Fear Mountain Shrine, Faye was already a Lv. 19 Cleric and by the time I reached the Mila Statue, she was Lv. 20 and ready to promote. However, I had to grind Silque to Lv. 20 just so she wouldn't fall off to literally only being a Warp machine (which she still kind of is anyway), along with a few other characters who were close enough to Lv. 20 to where I could do a bit of grinding to level them up.
Faye's ability to tank so well also means she naturally levels up faster without any thought because she actually defeats enemies by herself, especially since Silque can teleport her into the fray faster. Silque on the other hand wouldn't be getting a lot of EXP at all if the Bonus EXP system wasn't a thing, that's the only thing that keeps her from being one shotted by literally everything.
In general, I just find Faye better because she can play way more roles than Silque ever can. Silque's Warp is great, but other than that and basic heals for your back units, she isn't going to be doing a whole lot, except for occasionally Res tanking. Which she is much better than Faye at, but it's less of a boon because of the severe lack of magic based enemies in Alm's route for most of the game.