The Darnell has a GRP hull, which is considerably tougher than the Robbe ABS construction. The Robbe is a little easier to transport, as it splits radially into two halves, an important consideration if you drive a small car.
The Darnell boat traditionally uses boxes with bolted acrylic lids and a rubber gasket for waterproofing the electronics. Some were set up as static divers, some set-up as dynamic diver. The Robbe uses a cylindrical hull to contain all the equipment, with o-ring seal about a third in from the stern. This is a much better system IMO. As standard they are dynamic divers, but can be easily converted to static dive using an Engel piston tank. The Darnell can be retrofitted with a cylinder if you prefer.
Both boats have inaccuracies and less detail than the OTW and Engel Type VII kits, but then they are both considerably cheaper than either of those two models, and they are smaller too.