The eternal question.
Most important thing is to realise that they're not on different levels. Personal preferences completely dictate which one you should choose.
- Gear vs belt driven is a distinct difference.
- For use with console, Fanatec has greater pedal options (CSR, Elite, Clubsport, and even G27)
- G27 could be a more reliable wheel.
- Fanatec has on wheel setting adjustment, independent of the game.
- Plenty would argue that the G27 shifter is better than the non-Clubsport Fanatec options.
There's plenty of info out there. See if one particular attribute (or lack of) sells you. For me it was wanting belt drive.