I agree that both solar and windmills provide the ideal solution (both are very quiet and utilize the resources we have plenty of here in CO - sun and wind. I'd be more inclined to go with the windmill as we always have wind but some days aren't sunny enough to get much charge on the solar panels (then again - I may not have any idea what I'm talking about. I'm sure plenty of folks on this site will tell me how successful their solar panels are for them. But, what I've seen first hand with my buddies solar panels during hunting season is that on very cloudy days they don't charge his batteries as well.).
The problem with using a windmill isn't the lack of wind, but the moving parts; they will eventually wear and break.
I think the best solution is a combination of both solar and windmill.
ETA: Ref Stuart's comments below: The good thing about wind power is that the wind loves to blow here @ night when you can't get the sun to re-charge the batteries. If you have wind power, you can use the pump at night without too much of a drain on the batteries.