Wich Nino Should I Promote?

An Atk boon is probably more useful in that case than it is on a Nino, but if I could feasibly get their speed up to a usable level, I'd still probably go for a Speed boon. It'd probably take a lot of investment though. Otherwise I'd do an Attack Boon with a breaker skill and hope for the best. I have a +Atk -Hp Gronblade+ Summer Elise, and it's really hard to get value out of her sometimes because she isn't fast enough.

Definitely cases where +atk can be considered, but after looking at the numbers I think I still prefer +spd.

On Olwen I definitely prefer +spd and blade tome. If you're keeping Dire Thunder then probably +atk, but I think I prefer her with blade tome over DT. 23 neutral atk, is it? (My memory might be off) with 13mt and +30 from the blade tome's bonuses you're looking at 66 atk before A-skill, at which point I prefer making sure I double to 3 extra atk unless I'm fighting incredibly inflated amounts of HP (abyssal maps, sometimes GC upper infernal) in which case the extra damage might actually matter.

Similar numbers with Odin on a tactics team, except he's actually a touch slower so I want the extra spd even more. If he's only receiving +16 to +18 (full set of hones or missing one tactic) I think I still prefer +spd because he doesn't have cooldown penalty on his prf so the moonbow can cut quite hard against opponents with the res to stand up to atk in the upper 50s.