NFS is a "lighter" protocol, so as you've got something low powered like a Raspberry Pi, if you think you may be watching large (20GB+) films on it, then this might be better.
But, really, either will do, and it's down to personal preference. I am fully Windows based at home, so I use SMB.
Concentrate on getting 1 machine set up first (leave the Pi out of it for now), then it is as simple as copying the "advancedsettings.xml" file (and the "sources.xml" file, if you want both machines to be able to update the library) to the other computers.
So - on your existing XBMC box...
Essentially, follow
this guide...
Export your library to single files - this will create an XML file for each video file, in the same directory where your videos sit.
Share your media.
Install MySQL, following the instructions on how to set up a user.
Create an
advancedsettings.xml file, specifying the MySQL details (host, username, password etc), and be sure to set the "import watched status" part.
Remove any existing "sources.xml" and "passwords.xml" file from your XBMC directory (where you placed the "advancedsettings.xml" file)
Run XBMC, and you'll find you have an EMPTY DATABASE. Do not panic, this is desired at this stage!
Set up your source, but rather than selecting the local directory, ensure you choose your desired network sharing type first (NFS or SMB).
Once you do this, and add this source to the library, it will browse through all your files, and add them to the database, including your watched status (remember, when we exported the library as single files earlier, we saved this information)
Now - your first PC should be done!
Copy the advancedsettings.xml (and, if needed, sources.xml) file to the right place on the Pi, and start XBMC - you should immediately be able to access your library - WITHOUT doing anything else!
It's really not too difficult, the guide I linked right near the top of this post does outline every step in detail, be sure to read it through completely before starting do to anything!