There's no right choice, imho.
Robe is for slightly faster casting speed, mdef(?) and mana regen.
Light armor for better pdef.
Casting speed matters only for slow-casting heals like Radiant Heal and Brilliant Heal, that I use as secondary healing skills since Balance Heal, Progressive Heal and sometimes Panic Heal are more than enough to take care of most of the situations. At least in PvE.
The correct question is robe or heavy. To which, as Tiferet said, it depends on what you want, in pve cast really isn't an issue.
Personally I'm using 4pc vorpal robe with immo hvy bp. I have a full hvy set also but no ele on it and I need to check the differences of stuff for pvp. So far though the only pvp skills I seem to have been using are the instant cast ones.
Personally I'm using 4pc vorpal robe with immo hvy bp.
I should have thought about it... Oh well, I'm still in time :p
It's a pity L2 armors aren't well graphically designed to blend into each other (except for Nightmare robe which, mysteriously, blends rather well with Immortal robe gloves), at least in GW you can have your own set thanks to customization options like the colours on armors.
From my experience, you can stick to robe set, you aren't really involved in direct combat situation. However, you *may* prefer to have a backup heavy set for dire situations, like tank losing aggro... or the Bamonti in Harnak. They hit quite hard on healers (and they're specifically designed to target healers if not aggroed).
:( Ok, I'm currently wearing Moirai Light, so I should pick up robe with diplomas?
Robes look kinda bad on kamael... :p
Personally I'd sell the moirai for as much as you can get then buy 60*60*60 Vesper glove/boot/helm/stockings from the Auction for as cheap as you can get, you'd be surprised but with persistence you should be able to sell the moi for the cost of the vesper.
Then get an immo heavy BP for the diplomas. That'd give you decent elemental resist and p.def for next to nothing.
Its more like "Robe or Heavy?" IMO. But Robe wins. There are more mages then melee currently, or at least they do more dmg. What you need is mdef and casting speed for PvP.