I have both a Zrest and a inflatable pad. As Footslogger said, the inflatable one is more comfortable, but the Zrest is not prone to puncture. After having my inflatable puncture while on the trail and in a shelter, I quickly converted to a full length Zrest.

From my perspective if you spend any time in a shelter or on hard ground, definitely get the full length. The additional length is well worth the diminimus weight differential. I say this because I find my ankles and heels get stiff and sore if I don't have some padding.

In the winter I love my inflatable - esp if I'm tenting. It adds additional warmth versus the zrest.