I like my Silvestri but haven't gotten to use it much as I'm waiting for the lenses. I will be using it with the p45. As with the Arca, I am having to tweak focus on the sliding back. Believe it or not, Silvestri is nice enough to send a technician to Zurich to get pin-point focus with a laser gadget.
The advantage of this system is the possibility to use a sliding back; and, with the addition of the flexibellows, tilt. Longer lenses do not need a bayonet mount with the flexibellows. So, it is almost like a 6x9 LF but surdier and more field-usable.
When you call Silvestri, you first get a nice lady who will quickly connect you to a top technician (also very nice) who will take the time to get you set up with the right lenses and accessories. These are the kind of people I like to do business with.
I looked at the Alpa. They are down the street from me. It is nice to look at, but without a sliding back or tilt (or even rise) it is too limited for me.