Since I was in the market for a used truck a few months ago, I did both Carfax and Autocheck out of curiosity sake. I had leftover reports, so out of curiosity I ran my extisting truck I was planning to sell as well as one I planned to keep.
Autocheck was hands down the the more thorough of the reports on the vehicles I ran, including some dealer services that showed up in the report. Neither was fully complete - as an example, both listed my '99 F150 as a 1 owner vehicle, when in fact I was the second owner. Both correctly reported the vehicle as a "rental" and "fleet" vehicle, as that was the original use. Don't let that scare you away though, rental can mean rental car fleet or in my case, a company car through a company specializing in this type of rental and assigned to one driver and well taken care of.
If chose only one, in hindsight, I'd take the Autocheck report, even if not fully complete.
Nothing takes the place of a competent inspection in person or by a skilled mechanic though.