Between those two, Saika is the more unconditional one (8% damage for keeping Perfect Attack timing), but it is weaker. Both of Yamigarasu's Potentials require a bit more upkeep, though they give you a better damage multiplier (Wings of Twilight: 14% for Prefect Attacking in PA combos; Garland Guard: 15% for Perfect Guarding 3 times and not taking damage outside of a Perfect Guard).
http://4rt.info/psod/?3Oe4M (Using hypothetical numbers for character stats and PA to highlight differences, etc...)
Hmm I see. That’s a lot of information to unpack a bit. Is there a different option out there? Idk these are the ones I feel like I’ve seen mentioned once or twice so I figured I’d ask.