I use neither, but there are few things i can clarify.
2. Umbrel seems set up to use TOR as an option, whereas Start9 seems to have TOR on by default. I don't really want TOR running all the time, and from what I read, bitcoin core doesn't use it anyway. There is probably a way to disable it in Start9 from the command line, but I wonder if that will break things.
Bitcoin Core actually support Tor, see
https://github.com/bitcoin/bitcoin/blob/master/doc/tor.md. But you need to check with Umbrel or Start9 whether you need to configure Bitcoin Core manually to use Tor.
So you can turn TOR off (I'm not sure how, but I can probably do it from the command line) without breaking anything? My internet is pretty fast, but I had a data cap that I just paid to get rid of, so I should be good.
I'm not sure what exactly you mean by get rid of data cap. But FYI, current Bitcoin blocksize is about 700GB.
If I can do that I am leaning to Start9 over Umbrel. From what I can gather on the internet, Umbrel has more layers of containerization than Start9, so will probably be more complex to troubleshoot? I guess I could go with a basic Debian server, but I'm more interested in learning about Bitcoin than Linux sys admin.
Looking at some discussion at Start9 website, it seems Start9 also use docker (one of containerization tool). Here's an example,
https://community.start9.com/t/docker-error-no-space-left-on-device/2875.