I suggest you either buy a cheap set, so if it doesn't work out it doesn't matter, or go and try out several at dealers/manufacturers.
The tricky bit seems to be to find good headsets and get them fitted right. The model of helmet will also have an influence on sound quality.
I tried a PMR radio kit from Conrad Electronic in Germany and ended up sending it back, unusable. Some other people seem happy with it, though.
If you are always on one bike I wouldn't buy radios. Buy something that's linked. No interference and better sound quality.
There are also Bluetooth headsets appearing now: no wires!
If it uses batteries make sure you can use standard AA or AAA NiMH batteries, NOT proprietary accus. Cheaper and less hassle in the long run. (This counts for ANY gadgets you buy.)
I'm in the process of rigging up two old Baehr CB sets on our bikes. German users swear by them, but their gear is hellishly expensive. I'll report back in the Comms forum once I have something to report.
Salut from Southern France, the bikers' paradise,