Which H!Henry's would be better?

He can still reach 48 Spd if you go all out, though. I run a +Res/-Spd Henry myself and he is really, really strong. Gave him the Spd+3 Seal and SMN support, and his stat spread is now 41/48/36/28/41.
That's 82 magical bulk right there, and very decent 69 physical. A-skill is Swift Sparrow, and he can get an additional +2 on all stats minus HP via ally support. Maximum speed reached is 48 with full armor buffs. He is, of course, running a blade tome. That's huge. Oh, and this team netted me 6 defense wins so far today xD
He also baits and kills almost all mages in the game, including reds, and I just had him tank and kill a Nino and Linde with buffs and support from Lancina earlier, then kill Lancina next turn. The only one he didn't take out himself was the Eirika, and she got taken care off by Effie...
Considering how good his magical bulk is, and now well he can still do even with -Spd, I would actually say keep the +Res/-Spd one since it helps him a lot, maybe more than the extra attack, even. At least in some situations. Or keep both for different build if you want.