Well Concealment Op has really good single target burst and control. Basically you can burn down someone in a stun lock if they don't or can't break it.

Lethality is a DoT based and has really good sustained DPS.

Both are good in PvP but I think Concealment is tops in PvP and Lethal is tops in PvE

I run Conceal Op is PvP to just lolburst someone down in a stun lock.

Lethality shines when you have support and are really good at kiting and using LoS. Not saying that you can’t 1v1 with Lethality, but it’s pretty hard when compared to Concealment.

While Concealment gets the God roll, Lethality has big heals to it’s name, and can function really well as a support class of it’s own, spreading dots and providing off-heals, cleanses, and CC where needed.

Lethality's only really good defensive is cloaking screen (which concealment also has and more). Lethality + focus fire = squish

Concealment is burst and burst damage is the only damage that really matters. You have to be able to blow up people before they can be healed, guard swapped, etc. You have to be able to lock someone down and do that big damage before they can pop their defensives to escape. Lethality is a DOT spec and DOTs can be purged as well. If I'm playing an assassin or an operative, Lethality is pretty laughable.

With that said, everything works in regs. Play whatever is fun. I've seen many Leth players put out 10 mil+ damage at the end of a game just with fluffy DOT spam.