Senn PC360-
http://www.amazon.com/dp/B003DA4D2U/..._df_B003DA4D2U
A few big to huge advantages over the 350s-
100 Ohm lower impedance(50 Ohm v 150 Ohm)- obviously for best results an amp is still needed, but unlike the 350s which are hurt a ton by lack of amplification, the 360s are happy with even on board sound(although, if you want good sound, you should have a stand alone sound card obviously). No matter what you do, you aren't going to get sound quality close to 600 series Senns, not sure why they built these with such high impedance.
Open- this is a big deal for me, although some would tout the virtues of closed. Much better sound staging with open, and if you do any moderate to heavy sessions(8-16 hours) your head won't be a sweaty mess with the 360s.
Music- The 360s appear to have the HD598s internals for the audio end(obviously with the mic functionality on top of that) and they sound *much* better across a wide spectrum of music IMO when compared to the 350s. Now to be fair, if you are used to using $50 headsets the 350s will roflstomp them into the ground, just the 360s seem to hold up much better when playing a wide variety of music(350s get muddy sooner, again, if you are used to low end headsets, this isn't something you are going to notice).
Mic/volume controls- The 360 has the volume control on the right ear piece, not a tiny little nob, probably about an inch in diameter with little grooves so you haven no problem adjusting it- ever. At first I didn't think much of it, but now when I go to fumble with the stupid cord on any other headset I miss is badly. Also a very nice feature, particularly if you use sound activated voice chat- pop the mic up and it audibly clicks, at which point it is muted. This isn't some chincy little click like it's going to break, the mic is very sturdy and it feels more like they made something in the headset just to make the noise when it got to the muted position.
BTW- PC15x/16x are nice for what they are, but those are entry level headsets- pretty much the sound quality on them is fairly on par with other low end headsets, they just have a vastly superior mic(have a couple of those kicking around, the kids still use them).