For the best reliability, I would stay with an actual Mac being the iTunes server.
Anything that runs on a NAS like the 5N relies on third-party solutions that don’t necessarily support Apple’s DRM and whatever changes they may decide to make in the future.
Running actual iTunes as the server will keep you from always being a few steps behind. Also, you can install any available server you need on the Mac, as well as transcode if necessary (with Plex, Orb, etc).
Plus, having the server side as a separate piece (computer) means you can upgrade the server for faster speed or bigger network pipe. On a standalone NAS, upgradeability is limited, if it exists at all.
You’re getting 25-30 MB/s, not mb (which would be 3.125-3.75 MB/sec), right?
Regardless, that sounds slow, unless your 5D has recently been set up and is still doing its internal optimization.
How is your 5D connected to your Mac mini?
AFAIK iTunes sharing is not a “hand off” of “Hey, go retrieve this directly from _____” to the client device like native Chromecast apps do. I don’t think the client device has knowledge of the physical location of the library, only what the sharing machine is exposing to it. I’d be happy to be told I’m wrong here though.
If I’m not incorrect above, using a 5N as the library location for the Mac mini would clog the communication pipe more than using a 5D. With a 5N, the Mac mini is accessing the iTunes library through the network connection and serving its iTunes clients through the same network connection.
With a 5D, the network connection is only being used for serving iTunes clients, accessing the iTunes library is through USB or Thunderbolt, so there’s more bandwidth available, and more importantly, more stable throughput on the network connection for the streaming.
Of course, if you have other uses for the 5N, as you’ve noted it might host your Aperture library, then it might make sense to go with the 5N and still use the Mac mini as the iTunes server, even though you might be clogging its network pipe.
With an iTunes-compatible server app running on a 5N, the communication is direct between the 5N and the client. However, you’re stuck with third-party solutions that tend to be “chasing” Apple anytime things change.