Having a Sorc @ 50 and trying different builds and having leveled a Powerpyrotech to around 32, I'd go with the Pyrotech as being the closest, but with substantial differences anyway.
A Sorc. feels a lot more like a SPriest with the DoTs, dispel, extrication(leap of faith), Force Lightning (Mind Flay) and there are a lot more skills that you can mirror up with a SPriest.
Powertech on the other hand is all about mobility and setting up burst.
Shatter combos? You can sort of do them in this game. Explosive dart and waiting for last second to rail shot while popping relics and getting other buffs can do some great burst (can't wait for thermal det. and 30% crit. mod. talent).
Kiting? You can run and gun with the flurry of shots and proc. off the 50% slow constantly or use Flame Burst if you're managing heat well. If things do come in your face, it's alright you can Rocket Punch or Flame Burst and refresh Rail Shot. Flame Burst also has 100% chance to proc the 50% snare if you're pyro.
Spellsteal mechanic isn't in, but you have some other cool things like Grapple which more than makes up for it and Spellsteal wouldn't be that important in this game anyway.
Kolto Overload is what you can consider like a mini evocate except on the move and less HP regain (15% heal over time).
CC? Well you can kite with the procs, a 4 second stun on a 1 minute cooldown, Carbonize (46?) which is a 2.5 sec stun AoE on a 45 second cooldown (can think of it like Frost Nova sort of). O and an interrupt on a short CD, just you gotta be in melee range to use it, but that's alright.
Flamethrower is your "cone of cold" like spell, but it's channeled, but does a ton of damage when you find a clump of people.
No "ice block" button, but the shield (25% damage reduction) lets you tank people while still dishing out damage and on top of heavy armor which a little extra damage mitigation is always nice.
Sorcs. only "mobility" is Force Speed which can be easily countered by a force charge, grapple, and other immobilizing effects.
Tbh. only thing Pyro is missing that was pretty trademark of Mage is Blink and Polymorph. No class has a spammable CC, so it's alright, and no blink, but who cares when you're a walking death machine anyway.
Powerpyro just feels more Frost Mage for me because of burst and the ability to be always on the move, so it still has a Frost Mage's mobility.
Hope that wall o' text helped because I too at first though Sorc. was more like Frost Mage, but it honestly isn't at all after playing Frost Mage in WoW for 3+ years.
EDIT: Also if you're thinking of playing a Merc, it is no way near as fun as a Powertech. Tons less mobility if Arsenal, boring missile spam, and no so good surv. when melee gets in your face. The Pyro tree for Mercs. isn't quite as good because you don't get 90% ArP rail shots, and resetting rail shot as a Pyro sucks since you have to stand still and cast some stuff and then depend on RNG further. To top it off, Mercs don't have an interrupt.