One is inflationary while the other is deflationary when it comes to supply. Additionally, one was arguably meant to be used as a currency - as the names suggest - while the other was meant to be used as a gas for non currency things. This doesn't answer your question directly but it does give some good food for thought.
I don't know about ETH and BTC, but USeD would be certainly a thing!
It depends, a currency's value (if software is open source) might be the inverse of the amount of political b.s. one must go through to adapt a new protocol internally.