I would recommend the one with the largest objective lens. They pick up more light, which makes night or twilight shooting and actual possibility, depending on the conditions.
If you can find a 40mm scope for that price, I'd definitely choose that.
I can see pretty well at night with the one I've got, and pretty much when there's any kind of moon out.
I would also get one with listed MOA adjustments. This will make sighting in much easier. Just fire three sighter shots, measure distance from bullseye both vertically and horizontally. Calculate clicks by (distance to move POI)/(MOA adj. * range expressed in football fields), and you're there!
As for mounting, you could get either an 11mm scope rail or a skinnier one, (for airgun scopes), and mount it to your gun using #6-32 machine screws. Most gun scope mounts are attached in exactly this method, though using calipers to get the rail exactly centered and aligned with the bore axis. I would recommend B-Square, but I don't know how the price stands up with your budget.
If you are looking for an overall good scope, I would recommend a Daisy Powerline 3-9x variable zoom, 32mm obj. lens scope. I've got one sitting on my desk right now because of a problem with it's mounts, but that's my fault and not it's. I installed it on an old gun with misaligned rails and I had to flip them around and bend them to get the scope along the bore's axis.
For spudgunning, I wouldn't look too much for accuracy unless you're using some sort of dart. Then, it'd be good to have a scope with some sort of Mil-reference in it, for on-the-fly drop adjustments, since projectile drop is a big factor in spudgunning. But that's probably way outside the price range you're looking at.
Just use the scopes dials to adjust for range if you have to, but just make sure the scope can hold a zero and clicks are accurate. You can do this with a simple test. Zero the rifle, then move the scope 10 clicks up, fire a shot, 10 clicks right, fire a shot, 10 clicks back down, fire a shot, and then 10 clicks back to the left, and fire. You should wind up dead center where you were originally zeroed.
Sheesh... that was long. Hope it helped!
) You'll get a better sight picture and the shape is more... well, scope-like and attractive.
