I've never used Infodog, but I have used Onofrio and Foy Trent as well as ones without an online entering service.
It actually has more to do with location than anything else from what I heard. Onofrio is supposed to be mostly west of the Mississippi River. Infodog is east (which would explain why I've never used Infodog). Foy Trent is...I don't know, just another service lol. Most UKC shows don't use any at all.
Foy Trent seemed to be more user-friendly.
Bottom line is you can't just pick one yourself. You have to use whatever superintendent the people hosting the show use. Most UKC shows don't use a 3rd party online superintendent at all, they use their own club members to accept paper or sometimes email entries. Same goes for lure coursing (around here, anyway).