Str is basically the equivalent of a 30% mod tacked on to every WS, but it will cap eventually.
There is an annoying thing about WS mods that the mods are actually reduced a smidge as we level up (the 'alpha'). So really, the 30% dex mod on evis is dropped down to almost exactly 25% and 4str=1 base damage usually, so str=25% also. so in reality, a 30% real mod is reduced to 25%, and str is just a straight 25%. So you can look at it as str is the same as a 30% mod, but caps. So a 60% dex mod like rudras, dex is 2x as potent as str for example. Evis/DE wiht 30% mods, 1str=1dex till str caps.
Long story short, point for point dex=str on evis for raw base damage. BUT, str caps and dex never does. You can have 10,000 dex and adding more will still increase damage. The crit rate from dex DOES cap, but not the base damage from the WS mod.
Str=Dex until str caps. Str adds a smidge of atk, dex adds a smidge of acc. Not enough to really worry about when comparing. Abyssea you will basically auto-cap str on most things from cruor buffs+gear with daggers. Crit rate will also be capped with RR+cruor+gear easy. Outside abyssea, Fstr is less likely (but still very possible) to cap.
When in doubt, use dex. Almost identical damage boost but never caps. (and synergy with SA far better)