The 3.5 ton horsebox has a number of advantages,firstly and most importantly it can be driven by anyone holding a standard driving licence.
Secondly it only requires an MOT each year undertaken by a Class7 garage - and every town has a few of these. Since the move to close VOSA stations I have found plating ever more difficult both logistically and in terms of costs. Thirdly they are usually easy to handle, very manouverable and excellent for squeezing into small spaces for access and parking.
On the debit side a 3.5 ton horsebox will not offer you much in the way of living, indeed it cannot as this would diminish the pay load to such an extent it may become a "pony box". Indeed the other disadvantage also relates to pay load.
The Master and Movanos undoubtedly score best on this but pay loads are still generally limited to 1 - 1.2 ton. Of course if you hold a pre 1997 driving licence you can have the option to up rate one of these to give a pay load of around 1.5 ton plus and still retain all the advantages of these small boxes, which incidentally often offer more comfortable large horse dimensions,and sometimes just as good a pay load as the 7.5 ton .