Personally I'm not a fan of the platform dumps, although we (I represent Top Notch Equipment and own Compact Equipment Store.com) sell them and could make a package for the the Boxer and the platform dump trailer.
Hypothetical:
The 14' dump trailer with a 4' platform and a 18' dump trailer are the same overall length.
The 14' trailer has up to 14' for debris.
The 18' trailer has up to 18' for debris.
The 18' trailer loaded with 14' of debris and still carry the mini
The 18' trailer CAN carry more debris than the 14' trailer.
The 14' trailer will never carry more debris than the 18' trailer.
The only advantage the 14' trailer has is you can dump the trailer without unloading the machine.
The 18' has the advantage of carrying bigger loads and could still match the 14' on the last load of the day/job to carry the mini.
How tall are the sides on the bed from the ground?
Have you considered the Avant? The biggest downside for me is the Avant currently can't fit through a 36" gate, but for me, that's not a big deal and we (my climber) have a 532 as well. The 420 Avant has a 3' higher lift height than the 532.