Hi Paul.

I was in a similar situation recently but hadn't used either. Got a demo copy of each and had a play. I found the Autodata much more intuitive and user friendly. However the esi did seem to contain more in depth information with regards to things like ABS, SRS service/repair once you worked out how to find it. Both had the everyday stuff. Technical data, service schedules etc. but the autodata was easier to read.
I'm sure the esi is an excellent product but I went for the Autodata as it had the information I wanted, all linked nicely together.

Hope this helps..Phil