twin ropes:
better for pure ice climbing or routes in the mountains that contain a fair amount of near vertical ice.
this i sbecause there are two ropes that are cliped into the peice so there is almost no chance of a total system failure and it provideds more protection from falling ice and or rock. if one rope cuts you still have one. your not stuck out on a ledge on the north buttress of Mt. hunter with night creeping closer(knock on wood).
Half ropes:
can be used for anytype of climbing whether its alpine ice/rock, trad rock, sport rock. they reduce rope drag, put less force on your pro when you take a whipper and they are light
the PMI fusion or verglas would be my choice they combine awsome strength and durability and they are light and have no sheath slippage.
Aidan
[This message has been edited by highclimb (edited 03-24-2001).]