Corrected.

Website says "now with Steadyclock FS", but it's not in the name, indeed.

My bad, typos! Corrected.

So I currently have the UCXII, and looking at upgrading to either a 802FS or UFXII. My question was about the preamps in those two models.

I would definitively get the UFX II.

Have a look at my interface comparison Excel, it will support you finding differences
https://forum.rme-audio.de/viewtopic.php?id=35156

I see a huge advantage being able to control gain and volume for Mic/Inst and Phones digitally.
Then those values can also be stored in TotalMix FX Snapshots and workspaces.

You will also like features like
- Autoset which is very useful to find a proper input level for Mic and Instr inputs and
- DURec (for stand-alone and backup recordings)

Also the display is very useful for not only standalone operation.

Higher Mic Gain has already been mentioned.
The Mic inputs are equal to UFX II, similar to 12Mic, this can be very useful in some situations.

Maybe also useful, the later MADIface (USB) driver also supports 32 samples at singlespeed like PCIe based products.
And if you would get e.g. ADI-2 Pro FS or ADI-2/4 Pro SE at a later point, then they all use the same driver, then you can access all those devices directly through this one MADIface driver.

2x MIDI I/O instead of 1 can also be useful, if you want e.g. to use one port for remote control certain RME products like Octamic XTC using Auxdevice (remote control via TotalMix FX).
Then you can connect all your other MIDI devices to a separte MIDI port to split control of instruments and remote control of RME devices, which support being controlled through MIDI.

802 FS, UFX II, UFX III have all Steadyclock FS (you can find this information in my comparison Excel as well).