From my personal experience, both are good to keep, water dragons need constant handling to make them tame whereas beardies are really laid back and take to handling well.
if they escape from cage, beardies are easier to catch, water dragons are fast, real fast and excellent jumpers.
Its cheaper to build a large beardie enclosure as the waters need a well filtered water area. (they defecate in the water) means adding a tank or pond and filter or pump to cage cost, also building materials need to be able to take higher humidity.
The waters being slightly bigger require a bigger enclosure.
Water dragons are tough and really resistant to bacterial infections. Also as youngsters they are less inclined to eating each other.
The beardie makes a better pet but i prefer the water dragons. My wife prefers the beardies, more interactive and better to cuddle.
these are only my personal views.