I'd say it all depends on what you want to run on your Start9 server.

If you just want to run a Bitcoin node, electrs for your custom backend, maybe a lightning node, password manager and maybe file storage you'd be fine with the Server One.

If you want to run all of the above plus LLMs, NextCloud for the whole family, media server etc. you might want to opt for the Server Plus.

Like I said, it all comes down to your use case.

I run a DIY rig which I bought refurbished and it does the trick for me.