Red Buffer?

It will mostly fall down to each player's preference. I personally think that Marth is better in majority of the situations as a pure support hero. His weapon gives +6 in buffs that can't be disabled by panic ploy and/or other skills that ignore buffs (like Sharena's new lance upgrade for example) and the range of his buffs is more flexible, so you won't be depending too much on strict positioning in order to get all the buffs.

Eirika has hone atk3 in her weapon, which can be further developed upon by stacking more buffing skills onto her. Definitely viable as well.

The biggest comparison (and most likely the only comparison) here should be the weapon, because that's the only thing that makes the real difference and in that case, Marth is clearly superior imho.

Edit: if you want to dedicate that unit to supporting an arena bonus unit, you might also consider Corrin. He only buffs one hero, but it's +4 to every stat.

First of all, I do not recommend merging a unit just for this bogus ruleset unless you enjoy trying to solo entire enemy teams with a bonus unit. Months of feathers is far too high of a cost for any units other than your favourites, whether your favourites in a general sense or your favourites to play in battle.

Second, if this is only about the bonus unit, there's no beating M.Corrin. +5 or +6 to all stats (depending on how strong the ally support, 1 space away or two) and he can stack additional drives and a rally.

For Eirika vs Marth, please see here.
https://fireemblem.gamepress.gg/q-a/marth-vs-eirika