Both kits are excellent, and detail is superb. Parts fit couldn't be better.
Marder III: The only concern I had was the open area under the gun carriage. Built OOB, there is no interior detail, so I boxed in the area with sheet styrene and painted it flat black prior to gluing the upper and lower hull together. Even with the gun and ammo bins in place, you can still see through to the bottom of the vehicle. This step is certainly not required, but was just personal preference on my part. Otherwise, no problems.
Mobelwagen:
I'm still in the process of assembling mine (almost completed the hull and running gear), but so far the kit is great, with no problems encountered. As I understand it, this kit is based on the more recent Panzer IV moldings to come from Tamiya, and seems to be more accurate, etc. I have the Eduard set for this thing, and it does add some additional detail, most of it in the gun. If you want to build the kit OOB, it will result in a pretty accurate model. The only real complaint I have is with all the punch out marks on the undersides of the fenders. Sheet styrene will take care of that, though.
These are great kits. Have fun with them!
Gip Winecoff