Marth's Best Boon?

IMO, you really need the +speed boon on Marth. And -hp is not the worse. I think its the IVs i would choose. So play with it and lvl up him. If you draw another that you prefer, you can still merge. Taking a character to lvl 40 is done in less than a day anyway.

There are arguments for both +atk and +spd.

+atk gives you more consistent OHKOs against Myrrh and flying Tiki, who you may not be able to double (for different reasons) as well as Fae, who sometimes will block a double (but then she will have surviving one hit) and M.Grima, who will pretty much never kill you (except for breath skill + glimmer) but who will hurt quite a bit if you don't take him down in one hit. Taking down an enemy dragon in one hit is especially good if you're using reciprocal aid because it conserves his HP for donating to allies. The extra atk is also quite helpful when fighting high-def greens such as Hector, who might otherwise barely survive two hits and QR back for a win.

+spd is good for avoiding doubles, especially if you use fury. 40 without buffs or merges, and he might not get any buffs because he's probably the one on the team giving them all away. Survivability is good on a support unit.