I'm currently using the Harbor Freight 2" semi-trash pump to empty my pond for de-mucking and repair from muskrats and crayfish damage. I debated between the 3" and 2" also, and decided that while it will take longer to drain, they both accomplish the same goal. Plus hoses are less expensive and cumbersome to put in position. Also carrying a 2" pump vs a 3" pump to my drain site is less strain on my back. So far, I've been very pleased with how it performs. I was worried because there is quite a lot bottom growth (I can't think of the name but I think it's string or vine algae or something like that), and I was afraid it would suck up this stuff and clog up either the strainer or pump itself. So to keep that from happening, I fastened the strainer (with hose attached) to a flexible (or rubberized) tray I found at Tractor Supply. It's around 2' long x 16 inches wide x 6 inches deep. I drilled a lot of holes to allow water in and to let water out when removing from pond. I used zip ties to secure strainer and hose to the tray. I also tied poly rope to tray so that I could go on other side of pond and pull strainer and hose into position in pond. The tray has performed exactly as I'd hoped. I'm new to PondBoss and I would attach a photo but I can't figure out how.

Now, if time is a concern, a 3" pump will move that water out much faster. If you have a large pond, that is the best way to go. My pond is just under 1/2 acre and the 2" pump works fine for me. I will say that if you live a considerable distance from your pond site (like I do) and have to drive to it each day, a 3" pump will result in less time baby sitting of pump and fuel level.