Try before you buy. David Clarks have a good rep, but I don't like them, prefering a Peltor (which IMHO are excellent). If you do plump for a more expensive model, look at some of the in ear types. They use the foam ear plugs fitted with small speakers and a lightweight mic. Apparently very light, have excellent noise attenuation, are not affected by wearing sun glasses and don't get very sweaty. They will make you look like a call centre operative, but so what, you are not going to be wearing them in the bar!! A useful thread
http://www.pprune.org/forums/showthread.php?t=249883 here on sunglasses and headsets, but has input from someone who has worn in ear types. There is also an interesting piece about Bose headsets being inadquate in twin turbine helicopters. £750 down the drain
