My thoughts for whatever value they have - if your primary purpose for owning land is to have a pond then I would recommend build. I am sure there are downsides to building but my pond was here so I can only speak to buying an existing pond. If you are going to buy a place with a pond determine what was the primary purpose of the pond for the prior owner. Maybe it was a fishery and has been managed well but maybe, and probably, not.

The upside for me in buying a place with a pond is that I know it is tight as a drum and holds water, it never went dry during the drought of 2011, and it has a great watershed. The downsides are it was a 1-1/4 acre stock tank that also provided a place for the owner to also occasionally wet a hook but its purpose was not a fishery. That great watershed coming from north, south, and east.....well, it literally brought pasture loads of manure with it for a long time (at least 20 years probably more like 30). I wish it was bigger and the land and watershed can support it but given the cost, complexities and patience required, as long as I own this place I am going to have a 1-1/4 acre pond. It took a couple of years of fishing and keeping records to work out a stocking plan that hopefully will work with what was already living in the pond. I can't just say I want this fish and that fish and go to it. I have to be mindful of what the pond already is. Don't get me wrong, I love my pond and am getting a lot of satisfaction in turning it around and seeing it improve both aesthetically and as a fishery. But, the bottom line is, this pond was not built to be a fishery and so I have had to adjust my goals and be flexible.

So to me, if your primary purpose of having land is to have a pond - build the pond. If you get a place with a pond then understand the history of the pond and be ready to compromise.