Came across a well while soloing and did a little bit of number crunching.
I caught (or attempted to catch) 100 fish from the well, and then 100 fish from the pond.
I'm Prof level 14, and was using a lvl 15 +21 durability rod the whole trial. So I can catch Tier 1 to Tier 4 fish.
I did not count an escaped fish towards the total of 100, but I did count when I had to run away to save my rod.
"Useful" fish is subjective (of course) But the criteria I used was fish I normally keep to cook and resell. This does not include 'junk' Prof-raising fish.
First the well data:
Caught 98/100 fish, had to run 2 times. On 9 occasions, the fish got away (not counted towards the 100 total)
Total clicks: 889
Total prof gained: 570
Average tier fish caught: 2.91
Total amount of Life on fish: 3648
Total amount of mana on fish: 875
"Useful" fish: 25
*NOTE: I caught zero tier 1 fish in the well. I like this

And the pond data:
Caught 99/100 fish, had to run 1 time. On 7 occasions, the fish got away (not counted towards the 100 total)
Total clicks: 917
Total prof gained: 595
Average tier fish caught: 2.27
Total amount of Life on fish: 4036
Total amount of mana on fish: 274
"Useful" fish: 18
*NOTE: I caught 34 tier 1 fish at the pond. Not a fan

So, basically:
The difficulty level of fishing is relatively the same.
The fish are much better QUALITY in the well.
I was wrong in my earlier assumption, but that was in a full group that was doing wells, and only got a couple fish in.
If you're a fisherman and want better fish, Find that well!
edit: the useful fish from the well were more useful (quality mana fish as opposed to mediocre life ones from the pond)This post was edited by Mphyoi on Oct 18 2010 11:26am