I have 'crunched the numbers' on paper then tested my findings in SL, by monitoring several thousand casts in the last 2 weeks. (Did I mention I'm a numbers junkie?)
Once you get above level 5 it is a somewhat predictable XP result, when averaged over time. The bare number is roughly 1.4 XP per cast, but obviously this is subject to wild swings over short time periods. Consequently, a 30-min or even a 2-hour contest is going to be a matter of luck, and nothing else. Over longer time periods, the swings average out. Over a thousand casts, the hot- and cold-streaks balance out and the average does hold around 1.4 XP per cast. Over a week-long marathon, it's going to boil down to who's there the longest and make the most casts.
Consequently, IMHO, the only advantage autocasters provide is if you need to get a drink, head to the washroom, or if you tend to get more involved in the conversation than the fishing and keep forgetting to cast. Which I think are nice advantages of course - whether contesting or just fishing for levels / points / customs / etc.
I also do a lot of building, and like Judi, I appreciate that I can build and fish at the same time

Cheers!
-Atashi