I summarized the features of the new UFX III here in my blog:
https://www.tonstudio-forum.de/blog/ent … iii-en-de/

Further differences between the devices you can get from my Excel file here:
https://forum.rme-audio.de/viewtopic.php?id=35156

The AKM converters could be of special interest to you that are well known from the ADI-2 Pro FS series of RME reference converters.

The headphone outputs are of high quality in both units.

Other advantages of the UFX III
- doesn't need breakout cables at all (UCX II needs one for AES and SPDIF)
- no shared ports for DURec and ARC USB (for stand-alone operation, to be able to use DURec and ARC USB in parallel)
- 2x ADAT I/O, therefore no channel loss for 8-port devices connected via ADAT at double speed (due to port multiplexing)
- MADIface driver has a minimum of 32 samples ASIO buffersize @single speed, comparable to HDSPe driver for PCIe cards
- MADI offers good possibilities to expand in the future if needed

Should you intend to add an ADI-2 Pro FS R BE in the future, you have different possibilities to connect your monitors.
If you connect the ADI-2 Pro FS R BE to the UFX III through AES, then you can connect your monitors to the ADI-2 Pro FS BE's AES out port. The ADI-2 Pro switches then to AD/DA mode, and you could make use of dynamic loudness for your monitors connected to AES. The headphones could use all the nice features of the ADI-2 Pro.

Illustration of the setup

To make things clearer, what could be done if there is interest to integrate the ADI-2 Pro FS into the setup if you want to use its special features (different D/A filter, PEQ, dynamic loudness, crossfeed, sample rate converter, ...).

Applications:
- TotalMix FX
- DIGIcheck NG
- ADI-2 Remote (+) comfortably remote control of all of ADI-2 Pro FS's features via USB2
- RME connector: remote control for RME Preamps via MADI using (MIDI via MADI)

PC---------------------------------------------------+
|                                                        USB2    \                             
| USB3                                                            \                             
|                                                                       \                           
|         +---High-Quality analog I/O                    \ (+)                               dynamic loudness possible
|        /                                                                \                                   /
UFX III---AES OUT-------------------->AES IN---ADI-2 Pro FS R BE---AES OUT------------------>Active Monitors
|   | \    \----AES IN<--------------------AES OUT---/                          \-----Ph3/4------------------->Headphones
|   |  \                                                                                                      \
|   |   +2x ADAT I/O: connecting to e.g. preamps,                                    using all features of the ADI-2 Pro for headphones
|   |           converters, TV, HiFi (can be used as Monitor B)
+-+ MADI:
        - 64ch @single speed, 32ch @double speed
        - fiber optic cable with galvanic isolation
        - up to 2km cable between each of the devices possible
        - multimode fiber cable (OM3 or OM4 standard)
        - ideal for device distribution across rooms

Clock master in this scenario, UFX III.
Regarding ADI-2 Pro FS as clock slave: although UFX III has an FS clock and SteadyClock FS would already remove any jitter from the digital connection, ADI-2 Pro's special design is, to use its internal FS clock for the final D/A conversion.

In this setup, you still have two ADAT ports free for connecting devices via ADAT or optical SPDIF with galvanic isolation. New with the UFX III is, that both ADAT ports can be switched between ADAT and SPDIF protocol.

In total, more "headroom" / flexibility in use and sustainability of purchase.

Ok.
I mentioned different things that might be interesting for selection.
Did this information help you already for the selection process, or what is missing from your perspective?