Which refinement for Marth?

I think the drive refinement is the better option in a vacuum, but if you use him as your main offensive treat +spd would give you more combat power. Really depends on how you use him

The +spd refine is tempting, that’s for sure, but I think +effect is still better. Being able to provide so much support just by being there is amazing. It also looks more fitting for him, since he’s meant to be a leader that cares about his army rather than about himself(not a huge reason, though, it’s just some story stuff). +spd might still be better to make him a better dueler, though, in case you use him as your main damage unit.

Spectrum Drive is just too good to pass up. No one else can do it. My rule of thumb is to always give the effect refine for any unique weapon. So things like Ridersbane+ don't count, even if it has an effect refine. And if he runs the Drive Def SS (only drive seal so far), and another drive skill in the C slot, likely spd from a 4* Tailtiu, he's going to be buffing so much for his allies. Not to mention if they have bond skills and whatnot.

Speed refinement is fine But Divine dew is very rare to get and is very limited so I would suggest to go for the effect variant above all else, I recommend to upgrade every legendary weapon with the effect since most of the effects are very good while others are very niche in their own ways.