To keep this general, here are some things to think about:
DAS: Direct Attached Storage - Thunderbolt, eSata, USB 3.0, FW800, FW400, USB are the rough pecking order. FW400 is better than 100 MB ethernet, and FW800/USB 3.0 are equivalent to Gigabit Ethernet.
NAS: The network matters - latency over WiFi can be a big bottleneck as can the controller chip / lack of cache in less expensive NAS.
Think in terms of the bottlenecks - For streaming a song, AirPlay shows you what to expect - a slight delay, but the system can buffer the music to overcome little lags and delays due to network burps. DAS is much more responsive - especially for little file input/output as opposed to streaming a large file or song.
NAS does offer some nice features like cross platform sharing and with a good network can be shared effortlessly with several clients that don't mind sharing the speed of the device. DAS is designed for one user and optimized for speed - especially for database workloads like iPhoto.
NAS can be harder to back up, where DAS works easier with whatever backup program you use on your mac.
Your iPhoto database will be the last thing you want to move. It has no provision for splitting the large files from the database files and the latency of reading / writing to the NAS might make you feel the program is too slow. Try it out, but don't be surprised if it's the least responsive by it's design.
You want programs that can use local database and thumbnails on the fast SSD and store large original files on NAS or slower storage.
Aperture works amazingly well if you find iPhoto too laggy when stored on the NAS.
I would start with the iTunes media folder - move that to the NAS and see how you like the performance.
Then get a tool like WhatSize or DaisyDisk so you can quickly measure what folders on the SSD are largest as you prune things down.
You might find you don't need all the audio loops from Garage Band or other surprising folders that make a difference in what you want to bring along on the SSD.
Have fun with the tweaking - you'll learn how things work. Also, it won't be long until ThunderBolt storage is more available - your NAS might even have a DAS connection for when you really need some speed to access / move files to the NAS.
Don't forget to back up your NAS. CrashPlan might be a great thing to try for that.
Best Answer
iTunes keeps a lock file in the directory to see if there is an instance of iTunes running - only one is allowed at a time.
Similarly, if you're using the same directory, each version of iTunes will overwrite the library file, so trying to maintain two different sets of music within one tree isn't really possible. At least, that's what happened last time I tried this.
You're better off having Home Sharing enabled, and then having one Macbook at a time run iTunes pointing to the library.
The other can then use a blank dummy library, and play whatever the first is sharing through Home Sharing. Either can run in each role, just hold down Option at start-up to decide if you want to use.
Personally, given a family with a bunch of iPhones, AppleTVs, and laptops, I have a single iTunes server machine that is always on, with a very large library. All music is in my iTunes Match account, all videos can be synced over the air through home sharing, and wireless syncing. It's the closest I've found to an iTunes server device.