I say CoH because you'll likely want the resists. If you need the FCR, a Vipermagi is always a solid option too. Otherwise though, Nightwing's Veil (helm) and Death's Fathom are probably the best choices for the head and weapon slots, because of the damage boost.
But FYI, the bolts from IceBlast are a lot slower than many other projectiles (for example, I'd guess that Fireballs travel across the screen ~30% faster than IceBlasts), and there's no 'splash' effect, so hitting your targets (especially if they're highly mobile) might be a little more of a challenge than you'd like.
That's why I never fully tried this idea in PvP (although I probably will someday, and I have the level89 IB sorc for it already ). But I really like the idea of an IB sorc for PvP - especially since you can get damage comparable to Blizzard (at ~4x the casting speed) - but I think the projectiles are just too slow to be very viable.
I do know, from leveling my IB sorc, she's super-powerful against single targets (~7500 damage, 8-frame spammage, and Cold Mastery on top of that :shocked - and most monsters are easy to hit, so it works decent for PvM. But against semi-intelligent opponents...
I'd only use ormus if you have one already and it's + to iB or another skill you'll be using. And I'd ONLY use it vs hammerdins/melee characters where resist does NOT matter. For general public duel games I'd highly reccomend CoH over ormus, as already mentioned the resistance are important pvp, ormus is basically situational only imho.