The N20 and U2 are priced similar.
The Aurender is a server function which has facilities for a hard disk with music files on it, SSD or Spin, no matter really. For large library like > 10TB, the internal drives on the Aurender are way too small, unless you want to copy from a main library, can be tedious, not for me. Both units can accept external drives, but these are noisy and mechanically not reliable, build up too much heat for my liking.
Larger libraries need to managed well, so the Aurender approach is a trade off, the older models had drama with power outages causing corruption, but these days aurender addressed power rails with supercaps... not a s good as a UPS, but would not have one near any audio component.
The Lumin can cater for large libraries, all it does for the native app is to index the files on a server, and pull the files from NAS, PC or PC that's away from the Lumin.
Server software manages library, the Lumin just plays. For the moment the Lumin is the end point to Audirvana which resides on a Win 11 i7 64GB RAM 2 x 16TB HDD PC The computer is adept to managing the library with no issues, has the horsepower to do so, so the streamer doesn't have to worry about any processing, metadata etc, just play the music.
Isolation is by EtherRegen, & SilentPower LAN iPurifier Pro with a mix of Cat5e, Cat8 cables.
Often thought about the Aurender, but it misses out for large libraries unless I'm missing the point on how their conductor application works. Lumin are very responsive on correcting bugs and are leading protocol implementation ahead of Aurender.