I run both.

My Start9 is my Bitcoin and Lightning node. I run everything Bitcoin related on it, and nothing else. So Bitcoin, mempool, electrum server, DATUM, JAM, UTXOracle, LND & CLN lightning nodes, Alby Hub, and a watch only wallet in Sparrow.

My Umbrel is my homeserver, running Nextcloud, Photoprism, Calibre Web, Audiobookshelf, Jellyfin, AdGuard, a Synapse server for a Matrix community, and various other non-Bitcoin related tools that I use on a regular basis.

Why I set things up this way:

Currently, Umbrel has a much wider selection of apps and is easier to connect to remotely, since it has Tailscale and Nginx Proxy Manager alongside the Tor option. Meanwhile, Start9 has a good selection of Bitcoin related apps, but not a great selection outside of that, and until we get StartOS version 4.0, it doesn't have any option for connecting remotely except Tor and Nostr Wallet Connect; the latter of which is only useful for connecting to your Lightning wallet. Additionally, it's just a good idea to run everything Bitcoin related on a separate device than you run everything else to reduce the surface area of possible compromise of sensitive information.