RME is known to have transparent sounding Mic inputs. So you can concentrate on the features, like e.g. gain range and what max input signal is supported (also SNR and THD) where SNR is for all products already on a high level. A little difference in SNR and THD is unlikely to change the sound.

12Mic and UFX+ have more or less the same mic inputs with the same gain range of 75 dB.
Octamic XTC is a little older compared to 12Mic, UFX+, but nevertheless high quality, simply slightly different design.

12Mic, UFX II/+ have a higher gain range and support a higher input signal.
Octamic XTC has lower gain range and PAD for 4 of the 8 Mic inputs, the other 4 can be switched to act as Instr inputs.

The advantages of the Octamic XTC vs 12Mic are
- fits better into the MADI 8-port scheme, thus RME MADI features like Delay Compensation, Auto-iD, Auto-CA are available
- Auxdevice support is available (remote control through TotalMix FX)
- has even two phones outputs
- supports Audio through USB (class compliant mode), can be used in CC mode as preamp for Windows and iPAD
- Has ADAT and 4 AES ports
- can be used as format converter between ADAT / AES / MADI

12Mic
- higher port density, little lower cost per mic port
- latest design of Mic preamps like UFX II / UFX+
- Supports MADI and AVB
- Can integrate MADI devices into an AVB network
- Mic input 1-4 are switchable to act as Inst input
- Auxdevice support promised since longer (most likely because of Corona and all of its side effects..)