This is a useful PBO test report on gas alarms. I hope the link works - if not go to the home page web address and you'll find the link in the "news" section of Nereus website.
Having read this myself I quite like the idea of the Dualwatch alarm as it combines a CO detector within the unit - the others seem to detect LPG & Butane only.
On gas detectors.... are you concerned or not about them being intrinsically safe or flame proof. It seems that in all the PBO tests and bits I've read no one seems to bother too much about the fact that you are putting electronic/electrical gear into a potentially hazardous environment.
If there's an explosive gas mixture you've got a bomb with the ignition in situ.