I always recommend a wide pad; i.e., 40" wide, such as those offered by Oware and Gossamer. Such wide pads don't slip as much if at all and the extra width cups around one's shoulders providing extra insulation and wind block. Also, it's darn hard to slip off a pad that covers virtually the entire width of the hammock. Put another way, a wide pad prevents the struggles that I've read are common with a narrower pad.
The only downside I've experienced is that they are bulky to pack.