The only one that is steel is the DMR, the others are alu
The Mega is a aggressive all mountain/enduro frame with 150mm travel built around 160/170mm forks. Nukeproof do a 4x frame - but that's a hardtail and and also alu.
Don't know much about the others, but they wouldn't be the lightest 4x bikes.
The new breed of enduro (or whatever the marketing people are calling them) bikes - Mega, Covert, Meta AM etc.. are pretty sweet for as a general do everything bike - tough and enough travel to rag down a DH track, but still light enough and right geo to pedal around all day.
These new Ragleys shown at Eurobike look pretty cool too, big forks with less travel frames, could also be sweet UK rides:
http://www.singletrackworld.com/2011/08/eurobike-2011-ragley-bikes-goes-big-wheeled/