The thing to ask yourself is whether you'll want to add hard panniers at a later date.
I had the Yamaha rack that fitted onto the grab rails, but then found that it wasn't compatible with the Givi Panniers as you need to take off bits of the back of the bike to mount the pannier brackets onto the under-seat pipes.
So it might be more expensive for just the box initially, but if you want hard panniers later, go for the Givi setup.