A river is an ecosystem subject to constant environmental element inputs. I would give the well primary consideration as a more consistent source of water. Are you transporting this water yourself? If that is the case the logistics may sway to the river if it is closer? Just because water is from a well does not make it good. I use city water.
Good on the pH pen, but don't get a cheap one. I did, and was lucky enough for it to croak while I could still apply the refund to an Apera. You will also want an EC pen, but cheap on those works better - pH measurement is a bigger deal technically. I mention the EC pen because you will want to know how hard your water is because it can affect your nute mixing. If it is over EC300, it uses up a lot of "space" for the nutes. The river water may be better than well water in that regard. I ended up using lake and creek water and melted snow after I found that my tap water was 360 EC and was buggering my nute mixing.
