If you've got electric available, then I'd go with the cheaper submersible. I'd also put it inside a 5 gallon bucket, tying it inside and put some sort of a screen on top of the bucket so it won't suck any junk thru the pump. While it won't hurt the pump, you'll soon tire of cleaning out the sprinkler head. Been there, done that (often). Sink the pump so the top of the bucket is under water by at least a foot, maybe more, keeping the bucket upright so it doesn't suck junk from the bottom of the pond.

If you went with the gasoline powered one, I think you might have to rig up a bypass valve to bypass most of the water back to the pond. For that area, you wouldn't be using anywhere close to 100 gpm!

25 x 50, you probably could get by with 2 fan type ocillating head sprinklers, or a couple of rainbird type impact sprinkler heads.

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