I have had the KH80 for 16 months. I have found several issues with them during this period that ultimately caused me to return them.
Firstly, the DSP software was unavailable until about 2 years after release and was so far only released for iPad (which you fortunately seem to own).
Secondly, when going into standby, the backplate would occasionally resonate which made a rather unpleasant and quite audible buzzing sound. Of course, turning the standby function off avoids this.
Thirdly, the backplate resonates quite audibly at about 270 Hz even at moderate output levels of about 80 dB SPL at 1m anechoic (eg. - 14 dBu sine with KH80 set to 94 dB SPL at 1m anechoic and input gain at 0 dB).
After discovering the third issue, I went to the store where I bought the KH80 to compare to their exhibition model which had the exact same issue and the store personal also mentioned that another costumer had returned a pair of KH80 for the same reason.
Also, keep in mind that the KH80 needs to convert the incoming analog signal to digital in order to process it and then convert the processed digital signal back to analog. Thus, you have an additional AD/DA cycle in your signal path. The additional conversions are very likely to degrade the quality of the analog signal from the ADI-2 Pro FS. Whether that is audible is a different matter but the reviews seem to suggest that it is not.
The KH120A, on the other hand, has a completely analog signal path.
A third option, since you have the Pro FS, is to use monitors with DSP and digital input (eg. Genelec 83xx except 8320) with the RME's digital output (eg. AES-3). For this, you likely need a different cable than a microphone cable.
Neumann states a minimum listening distance for the KH80 of 0.5m and a recommended listening distance of 0.75-2.0m while for the KH120 Neumann states a minimum listening distance 0.75m and a recommended listening distance of 1.0-2.0m. Given that you mentioned that you have very limited space, this might make you decision easier.
In terms of size, the KH80 has an external volume of 7.0L whereas the KH120 has an external volume of 9.7L, ie. only about 40% more than the KH80.
If you have not already done it, I recommend you measure the spaces where you consider putting the monitors and see whether either will fit and what the resulting listening distance would be.
If it is possible for you, I recommend you go to a (pro) audio store to compare the monitors that will fit in the possible monitor positions with some music/audio that you know very well. However, keep in mind that the acoustics in the store might be very different from those in your own listening space. Since reflections make up a significant share of perceived sound quality, monitors with controlled directivity (even dispersion) will likely sound the most similar between the store and your own listening space. If you are in the store, try moving around (left to right, front and back, up and down) a good bit and see how the sound balance changes compared to the sweet spot (main listening position). This is most easily done using pink noise.
The Neumann have well controlled directivity as do Genelecs.
Also, try moderate to high level (eg. 80-90 dB SPL at 1m anechoic) slow (eg. 10s 20Hz - 20kHz) sine sweeps to determine resonances and rattling/buzzing that would be much more difficult to hear with music or pink noise.