For your usecase, DS 423+
The main difference between these two models is that 923+ supports expansion unit. But you're unlikely to need that and even if you do, they are so expensive, you might as well just buy another NAS device.'
Look at this, this is the kind of expansion unit you can connect to 923+
This costs 60k. Essentially it's just a container for 5 hard drives, and costs more than 423+ itself.
To be clear, the 923+ does have a better CPU. But you're unlikely to need or use it. I have DS220+. I run 6 docker containers on my NAS and few scripts. It is a downloading machine as well as media server. And I have never felt the need for more horsepower.