Hi dolsen,
As for numbers of fish per cubic litre of water I would say the Vedder has more right now. The clay banks are a mess up there though, there is even one up above the slesse confluence now so the situation is worse. Th e river is also very busy so maybe not great if your wife is a begginer.
There are probablt a few less fish in the Stave but it should be a LITTLE quieter and the visibility should be fine since the dams/reservoirs trap sediment.
Not sure if that is what you were asking for but hope it helps.
Pat