Which Hector should I stay with?

I don't think it would hurt to keep both for arena assault and stuff, but if you really wanna give DC to Myrrh I don't know what to tell you.

Hector (+Spd/-Res)
52/36/28/37/16

LA!Hector (+HP/-Def)
53/38/21/37/24

Even with a defense bane, LA!Hector is still able to keep up with Hector in defense, and he has more HP thanks to the HP boon. He has more attack and more resistance. The only stat regular Hector beats LA!Hector in is speed, which is not important for him.

They are both good units even with less-than-stellar IV sets. so it boils down to figuring out the following:
1) which Hector do you feel works better for your playstyle? (subtle differences between the 2, but they are there)
2) regular Hector has a greater availability, so if you feel you must get rid of one he's the easier of the 2 to re-acquire down the road (and he'll be a focus on an upcoming banner).

I think that regular Hector has an edge over LA!Hector simply because he has both versions of Armads.
But i suggest you to wait a bit. I heard that BHB is coming wich means we will se Hector on banner relatively soon.

Both Hectors will outperform Myrrh in many situations. A +0 Myrrh will never get your arena scores as high as Hector will. Is your Myrrh +10 or going to be +10 eventually? The only units you should be cannibalizing rare and powerful 5* exclusive skills for are units that are going to be merged. Otherwise it is a waste.

The only thing Hector has on LA Hector is his ability to run Wary Fighter and Quick Riposte seal to omnibreak. (No enemy can double him and he gets to double everyone while above 80% health in most enemy phase situations.) +SPD IV isn't good for that set anyways, so sacrifice your regular Hector. He's in the regular summoning pool so odds are you'll get a better IV version eventually. LA Hector shows up on probably two banners a year and sharing focus with one or two other units so way harder to get.

I'd keep both of them. I like to maintain several different teams for different purposes, and I'm going to keep two Hectors: one for mixed emblem and a separate one for armor emblem. I get really sick of changing my one Hector's skills around every time I need to swap teams.

Also, why not try giving Myrrh double distant defense? Since she can fly, terrain is no obstacle for her and that would add 12 res & def for any distant threats she has to engage. Remember, distant counter won't help her with melee threats if that's how you primarily use her. If you want her to be an anti mage/archer/staff unit, beefing up her defenses will make it easier to bait ranged units into bad situations and let her kill them on your next player phase. And combined with her breath's ability to prevent doubles, she might even be able to survive a hit from a Naga tome/arrows if you're lucky. And giving her Aether would help her sustainability. Also, Heavy Blade might be another option to help her special trigger more quickly.

Hoping to get a Myrrh myself is she appears on this next Legendary Banner ^w^ Also fingers crossed for Legendary Hector XD