Is a light armor better for merc than a median or heavy armor or is def the only thing that counts?

They say that in pvp, a heavy armor is good, because it makes the merc attack a few frames faster after teleporting.

It's for an pvm a2 merc.

mercs get slowed by medium/heavy armor, but its your choice if you consider or ignore that. given that most of the time you reposition with tele and/or your merc gets a frw bonus from e.g. infinity, you might just focus on defense

defense matters pvm

and matters in pvp

melee vs melee for example

Eth sacred armor

I use a eth 14%ed Sacred Armor with Fortitude on my Insight/Infinity merc. This is quite nice but now I am looking for a nice alternative for the CoH RW on my Druid Merc. He uses a Flickering Flame helmet to compensate the -fireres from the Sunder charm. I have two bases left 15%ed eth Krkaneshell and 15%ed eth GH. So I want to choose the right one.