Out of those 2, I'd go grenth.
It depends on how much you need the adrenaline, whether the enemy is fleshy(and if they are, are they human) and if they're undead, I guess. If you're using balthazar to pump out more attack skills it might beat out grenths, but that +10ish life steal goes on every attack. I haven't done the exact math on this but I'm pretty sure that that will outdo the damage from burning+the extra attacks from adrenaline if you position right and use the right setup(burning is only like an extra 14-15 dps iirc, a IAS with a couple fast attacks, the extra lifesteal should outdo that, and can't be healed off). Then you get the disease on top of that. But if you're fighting undead that goes out the window, tear ass with balthazar's holy damage.
Now if you can get it, in PvE Dwayna is just awesome. Go D/W with SY! and you're like an imbaderv, every 6 seconds like 80 health flowing over your party. That comes with Anti Wipe(eternal aura), and a buttload of PBaoE(aura of holy might). Pick an attack and slap on pios fury and go whomp ass. If your attack is a teardown go earth, take stagger and dust cloak, if it's not, go wind with whirling charge and rending aura(each of these are the fastest recharging flash enchants, the next fastest are the 10 second ones in mysticism, if you don't want to spread the points to earth or wind).
This setup seems to be interchangeable with any avatar, you get a decently fast activation of the avatar's effects with some good nuking from AoHM, but it just owns with dwayna, you get badass healing, prot and damage on one bar. I went HMing nightfall missions with it, and I'm doing pretty awesome.
By that same token, I think people are really underestimating the potential of the lyssa avatar. Take the above example, but now all the FEs recharge in 3 seconds, you'll need to go wind for whirling charge's auto rip on hit and a tearing attack to get the stuff off you fast enough but damn... and in PvP.... Remember why they had to nerf the hell out of Fear Me!.....
Melandru is the weakest now, the extra health isn't necessary, and the armor, while nice, isn't either, not when you get the protections you can get in pve, and there's better ways to kill conditions in pve. Dwayna can probable outheal any damage they would cause, there's lots of ways to get rid of them partywide in the rit and pragon lines, and if you need to be immune to them, blindness, the only real threat to you, can be beaten with sight beyond sight.