Legendary Ryoma for Firesweep

They both can have the exact same matchups on offense depending on which seal you give them. +Atk is better for other builds, so I'd recommend using the +Atk one with the Speed+ Seal. Ryoma really doesn't have any good skills, so merging is probably your best bet.

Flyoma has so much speed, particularly if you run LnD3 or SS2 in your A slot that a +Spd may be overkill. With Flier buffs he'll be doubling nearly all the time. +Atk on the other hand gets you more leeway to proc Heavy Blade, so he'll proc a 2CD special every single combat.