Depends which tractor you are talking about - there is a big range in hub weights. The weld on plate would be maybe 15 lbs? Personal preference would be Cap bros light weight hubs.
The primary reason for weld on hubs is the propensity of some brands to break hubs under heavy load. But then it best have been a good welder, as I have seen more welds fail on hubs than cast hubs that have failed.