if you care about pvp at all, nais by a country mile 

I'd definitely go Nais for PVP, but if your primary focus is PVE, then Mikage.  

I had 13 books so I did both.  I noticed they are booked for their base form.  I have a feeling I am about to learn a surprise that I have to book their second form.   

I am parked in Classic Arena gold.  I hate Live Arena but its a necessary evil.  Nais and Armanz will make them think twice who to ban.  

Is Nais a high value nuker for Hydra?  I am stuck at Brutal.  

Yes, sorry to let you know this way, but mythics have to be switched to form 2 and booked seperately from form 1.

I don't have Nias, but I'll tell you from fighting him I'll take my chances with Cap'n Jack and ban Nias in live arena 100 out of 100 times.

Not much of a PvE champ really.  Insanely high value for PvP.

Nais' weaknesses are Bombs, Marius' Passive & Crowd Control.  

Depending on who else is in the draft, I will sometimes chose to fight Nais.  

Fortunately for you, the base form is the one you want booked, for both champs.

You really should avoid fully booking people though. Mikage, for example, doesn't really need her a1 or her a2 booked, so you could possibly have just used one book and been done with it.

For Hydra, Mikage's Base Form is the important one to book, but having Mikage's Alt Form fully booked for Hard FW is brilliant.  I don't use her Base Form there at all.   

So much Feral Nais now.  Which there should be