So it sounds like you might be saying there's really no innate advantage to one Fo over the other?
Well, there are innate advantages to certain FOs (see below chart and especially notice the highlighted fields). I would say use their innate perks / stats to cater to your preferred playstyle and select gear around that. Many of the super effective Ultima gear is equippable my many classes. Check out the link I sent you before coupled with the item database to see what's worth getting.
Though if we are wanting to avoid damage cancel and nuking is less effective, fomar and fomarl are going to be your better shooters and meleers correct me if I am wrong.
Those are my thoughts too! FOmar/FOmarl are great choices for FO melee/range damage (FOnewm too, but to a lesser extent). Best to choose a class that has weapons/armors/class perks that most closely match the play style you get most enjoyment playing.
I feel that FOnewearls' very high MST isn't enough to make her shine. Tech weapons with damage % multipliers (think Psycho Bridge, Psycho Wand, Sorcere's Cane, Glide Divine V00 and the like) are more effective at increasing nuke power than MST points alone. Her support boosts to Resta/Anti alone are outshined by FOmarl's. Penetrating megid is only occasionally handy, and there are a couple weapons that you could easily switch to get the same benefit (Demonic Fork, Ultima Reaper). Besides EP1, most enemies high such high element resistance that even with her 1.3 boost to the simple spells, she'd do more damage using powerful melee (Tyrfing, Vivienne) or range (Sacred Shot, Bringer's Rifle) weapons than spells for attacks to single enemies. In fact, probably her best strength is probably her ATA. She could most effectively use demon's weapons (Slicer of Fanatic, Bringer's Rifle) though.