Main differences are:
PC360:
Open acoustics for a more natural bass response (and it helps keep your ears cooler over long periods of time). As a result you (the person wearing the headphones) can hear environmental sounds more clearly and more sound leaks out of the headphones.
PC350 SE:
Closed acoustics. Less sound leaking out of headphones and more environmental noise reduction.
Collapsible headband.
Comes with carrying case.
I own the PC360. I think they sound great. Great bass response with clear and crisp mid- and high-range. Soundstage is good enough that it can approximate surround sound pretty decently using virtual surround software. Mic also works well enough for gaming/skype (friends tell me they can hear me clearly; anything beyond that doesn't really matter to me). My only complaint is that the wires to the 3.5mm plugs for the headphone and mic share a common sheath and only branch for the last 10 cm or so, so if your headphone jack is more than 20 cm away from your mic jack (like mine is) you're going to need to buy an extension cable.