Thermarest full length for vehicle camping, 3/4 for backpacking and a thermarest seat cushion that attaches to the frame of my alice pack which serves as a pad between my pack and my back. Thermarest, best outdoor gear since the protractor baseplate compass.
Register a vote for the Thermarest 3/4 lite. I have two, love 'em. Be sure to buy and carry the repair kit, saved me one time at Bright Angel CG, the in-field fix is permanent and has stood up for several years now.
I second the opinon diss'ing Playtypus. Now use a Dromedary Bag. I little heavier but much more durable.
Yep, Thermarest. Pretty hard to beat. Bought John's first in 1980. Neanderthal, eh?
But Thermarest can use some help. These days we use a $5 closed cell foam pad from
Popular to place between the T-Rest and the ground. Makes a world of difference. (And a sleep's, too!) Sharp edged rocks? No problem! Keeps the T-Rest intact longer. Wrap it round the T-Rest on top of your pack. Keeps mesquites, etc. from ripping your T-Rest. Staves off repairs all 'round. Doesn't add a lot of weight. Not enough for us to notice, anyway. Pretty plain common sense when you start thinking about it.
Therma-rest all the way. I use a full-size guidelite. I've found 3/4 lengths make me arch my back because my legs angle down off the pad, and I'm sore in the morning. But, I'm 5 foot 10 too. In my experience, 3/4 works as well as 1 inch foam width, but the one inch make a more comfortable chair. When weight's not a problem, a gidelite long pad is total camping luxury.