you cant go wrong with sonus sfx clay, its very soft and fine and if it doesnt pick up some containiments the first couple of passes, then just spray some more lube and go over that section a couple more times
i recently tried mothers clay for the hell of it and it works quite well, very soft and since its yellow, you can see if its time to change to a new side.
regardless of which clay you get, just make sure you use plenty of lube, dont want to marr the hell out of your paint
=D