Marth -hp, +speed or +attack?

depends, if you have a lucina go with +Spd and make him a bulky falchion support
if not go with +Atk add a breaker skill and use him as a budget lucina

I have a +atk -hp but i think +spd might be better with a fury build. His base offensive stats with falchion are 47/34 and with fury +spd it is 50/40 which is really good and his def is pretty good reaching around 31

I have a +atk -hp but i think +spd might be better with a fury build. His base offensive stats with falchion are 47/34 and with fury +spd it is 50/40 which is really good and his def is pretty good reaching around 31

If he's going to sit on a bench just take +attack and use him to nuke dragons in arena assault.
If you actually plan on using him later +S

i know the old red units r getting shadowed by the new ones but he can still be useful as a falchion user merge them since he doesnt have any unique skill and use the +SPD to reduce the doubles against u and give him
Ardent Sacrifice + renewal 3 for PvE content