ROCK on a NUC gives you a Roon appliance for exploring and playing music, and managing your music collection. Similar to a microwave being an appliance for heating or defrosting food. They do what they are designed to do, do it very well, and do nothing more than that.
Windows on a NUC gives you a general purpose PC that can be used for many things. It’s also a more complex environment that needs somewhat more care and feeding.
Roon Core on a Windows laptop is a great way to explore what Roon is, but personally, I moved it off to a dedicated device pretty quickly. First to a NUC running Windows and then to a NUC using ROCK. An appliance is my preferred way of having a Roon Core that just works.
going 8/16GB ram will future proof the library size issues. you definitely want an NVMe drive but 128GB is more than enough. if you can get a fanless chassis (AKASA) and if you want to run an internal music storage drive you might need the taller NUC options that support an extra SATA drive with connection hardware. Lib drive doesn’t need to be SSD bit its quite if you not need a huge library