Conversion quality (AD/DA) is very good generally.
RME is known for a transparent (no "signature") sound.

Newer products tend to have slightly better technical data (SNR, THD, lower converter latencies for A/D and D/A; see technical section of manuals) where differences are in most if not all cases measurable, but not audible in a blind test.

You should not only consider pure technical data when selecting such products like AD/DA converter.
You also have to look for certain product features.

The ADI-8 QS is an excellent AD/DA converter and one of it's strengths, besides quality is, that you can
- control everything at the device, you do not need a certain software for operating it
- can be upgraded to support MADI, but this module is quite expensive, better get a ADI-8 QSM with MADI module
- can be remote controlled either through MIDI or MIDI over MADI

But this converter has been designed for special purposes in a studio if you want to connect 8 analog channels of a mixer of a tape where all ports run at the same reference level on inputs and on outputs.

Newer products have meanwhile the flexibility, that you can set the reference level on a per port base like on a recording interface like the UFX II or UFX+. This might be a requirement of your project studio .. or not.
AD/DA converter with ref level settings per ports are e.g.: M-32 Pro AD and DA or the M-1610 Pro.

You can use smaller Recording interfaces like the Digiface USB, but it could be a disadvantage not to have
- WC (word clock)
- MIDI for any remote control options

Another question is how far away from the recording interfaces the devices have to be placed.
ADAT: 8ch @single speed, cable length 10-15m
MADI: 64ch @single speed, cable length up to 2km between each of the devices in a MADI ring of 8 devices (8x8 ports).

Some other devices support not only MADI but are also prepared for AVB, so if you want to move to ethernet based setups, the newer devices like M32-Pro or M-1610 Pro could be an advantage, but because of this they are more expensive.

I personally think the Digiface USB is too limited as it does not offer MIDI and WC which you might need at a certain point. Latest if you use Focusrite preamps, which do not have an ADAT IN port for clock sync, then its better to use WC (word clock) or you need to use this device as clock master, but what if you have two of those ?!

Better choice would be the use of a PCIe card like the HDSPe RayDAT if your setup / computer allows.
Because then you have besides 4X ADAT also 2xMIDI, SPDIF, AES and optionally WC.

Ideal RME setup would be IMHO:
- UFX+
- ARC USB
- ADI-2 Pro FS R BE for the monitoring section  (via AES or ADAT)
- M-1610 Pro as additional AD/DA converter (via MADI)
- 12Mic as Mic Preamp (via MADI; can be remote controlled later also by Auxdevice/TM FX)
but of course it's quite expensive.

With the ARC USB you control e.g. Main Out of the recording interface, you define whether this is an analog or digital port of your recording interface.

Question is
- how many devices you want to connect, which exactly
- what sample rates do you need, also double speed (port multiplexing robs 50% of ADAT/MADI channels)
- whether you need reflevels per port
- whether you need remote control features through either MIDI cabling or by using MIDI over MADI (if you have MADI)
and
What is your max budget for this...