SPDIF can run over optical, typically doesn't. ADAT can't run via RCA. It's optical or nada. SPDIF is even more confusing, because you also have SPDIF AES.
SPDIF is regarded as 'prosumer' and ADAT as okay, but not really 'Pro Studio'. Madi is seen as the more pro studio version of ADAT. But's it's waaay more expensive.
My system has quietened down a lot, since switching to ADAT. I'm very pleased with the set up.
Frontend. Honestly, not sure... mixing desk or 500 module? Good arguments for both. Allen and Heath mixers are very affordable and provide a very decent sound quality. Luckily I have nooo money at the moment, so I don't have to make the decision any time soon 🙂