If you are going two up at all, get the larger one. Pillions really like the grab rails. It seems to give them confidence even though they might not use them much.
The larger one also comes with threaded mounting points for a frame for mounting saddlebags - handy when two up camping when you've lost the use of the seat for luggage. Like this ....

It does change the look a little, but so very handy. There are other options.
Mal