Mixamp will certainly be easier, as it's all one DAC and amp.
Despite the latest version apparently having a weaker amp, I would imagine that it will still power the PC350's sufficiently. Although they are 150 Ohm, they are going to be efficient. It wouldn't make any sense for Sennheiser to make a gaming headset inefficient, because most people will likely use basic onboard audio, for the majority doesn't have anything in the way of decent headphone amplification..
I am rather sceptical though that the latest Mixamp will drive any headphone. The HD5xx series can be powered by a potato. It's not necessarily about how loud the volume is though. A guy who reviewed the latest version with AKG K612 headphones found the sound to be flat and lifeless, compared to Creative's E5, which he described the sound as being "full of life". They are inefficient headphones though, so not so surprising if the amp in the latest Mixamp isn't as good as the one in the 2011 model. As I said before, Ryan found that the 2011 model did a good job with the K612, so there must be some difference.
Any model of the Mixamp should do a fine job with efficient headphones. Most Sennheiser's are efficient and easy to power, except maybe their higher end HD6/7/8 series.
Something I should have mentioned before is that using a Mixamp or DAC and amp, is going a much better solution than connecting an amp (FiiO E6) to the controller, because as Psycho Sonny said, the quality of the Bluetooth connection is a weak link and also the DAC in the controller itself isn't going to be particularly good. Fine for simple comms, but not that good for anyone wanting decent sound.