My little bit of experience, I have used both Bose X and the light speed zulus.
Both are excellent, I have used the Bose on the Cl-605 which has a quiet cockpit (I think). The cockpit with the bose on almost becomes "too quiet". One downside is when the flighty come and talks to you, you usually wont notice they are there until you get a tap on the shoulder - are you listening !.
Light speed zulu in my opinion are a little quieter than the bose head set. The bonus with the Zulus is you can plug a MP3 player into the headset controller and listen to music - much more comfortable than sticking earbuds under you headset. The 80% automatic volume reduction of music whenever a radio call or just chating is very handy.
The other great feature is the bluetooth function to join your mobile to your headset.
The only negative of the Zulus is perhaps slighty greater clamping pressure - hardly anything.
I bought the Zulus, I was almost ready to by bose but the extra features and being slightly quieter won me over on the Zulus.