For the top case, I'd recommend you go with the Medium. It will hold one and a half helmets
Or you could put one helmet in the right case and one in the top case.
The large is huge and the small holds 1 helmet. I believe the lids on the top cases are removable, so if you want to use it to collect fire wood at your campsite or an impromptu drink cooler.
Mounts, it's personal. For me, the luggage rack location is best. It still leaves you the pillion deck for strapping a big dry bag, or to carry a passenger. If you mount it in the passenger seat location, it's handy as a rider's backrest, but getting on and off the bike means you need to stand on the pegs and swing your leg over the case.
I mounted my V2's on my 800GS on a 2 up trip through Colorado this summer and the cases were excellent. Of course, I wished I had more room like I did when I had the Pilot cases..
I will be using the same V2 cases on my next bike, the Yamaha XT1200 SuperTenere. I also need to post some pics of the 800GS with the V2's.
Here's the Large on my old bike: