The device that you are talking about for the QC15 is the UFlyMike:
Products You can read the multiple reviews online as well as I can about it - concerns include the lack of any passive noise reduction and that fact the whole thing dies when the battery goes flat. There is also a slight possibility of the mike unit becoming detached from the headset (which you wont realise until you try and speak). I haven't met anyone who has one but $225 (list price) seems expensive to me, although some online reviews are very positive. Ignore any mention of TSO - doesn't apply to private flying anywhere.
I've always been told you should get the best headset that you can afford. You wont regret it! Buying a cheap headset initially is seen my some as a false economy. You may aswell start as you mean to go on, although a cheaper headset could be used as a spare if you upgrade in the future.
The club I'm at has free headset hire so although I'm well through my PPL I haven't bought one yet. Better to spend the money on flying! The club headsets are David Clarks which are better than an HME 100 that I was lent, despite them seemingly having been around for ever. So if going for a passive headset, I would lean towards Dave Clarks - they are solid (the club ones are very well used but still going stong) and a well known quantity which wont let you down.
If you are going ANR (active noise reduction) the bose A20 still leads the way. Unfortunatly for us, Bose also leads the pricing table too. Lightspeed Zulu II generally comes in second to the Bose A20 although just about better than Bose X (older Bose model) on noise cancellation. I can't comment on other ANR headsets, although the Dave Clarks aren't supposed to be great in comparison.
Only other comment on Sennheiser's is that I have known them to occasionally be incompatible with other headsets in the plane. I can't remember the factors involved in this, but they include which type of intercom is in use in the plane and was something to do with hi verses low impedance.
A number of medium and higher range headsets now come with bluetooth for receiving phone calls and some allow you to stream music into the headset. You are likely to only use the bluetooth rarely while still on the ground or when you are a passenger. Since you can buy a cheap bluetooth headset for £15 - I don't understand why there is a £100 premium on them! Unless you have a good reason to go bluetooth, I'd lean away.
Personally - I'm going to save up and plan to spend too much money giving myself a treat once I get my PPL...