I use Auto Sleep, and as a backup Sleepwatch, mainly because I had purchased both. I like the layout in Auto Sleep, and I do like the option of the "Lights Out" feature where it will take the time you get in bed, and how long it takes to actually fall asleep. I find the data is quite accurate, to the point that I really don't care if Apple develops a sleep tracking app at all, this one works for me, and gives me more insight than I need.
Sleepwatch is a much more simple app, but still does break down your sleep patterns with good data, giving you a "sleep score" that indicates roughly how well you slept. I've found it to be pretty darn accurate.
Just my 2c worth.