For me the 8gb still be more profitable if you are planning to mine ethereum but if you are mining with other coins or not dagger-hashimoto algo then both all of them have same performance so you can choose all of them.
None. ETH is going to go PoS and after that there will be no profitalbe coin to mine
Forget about it. It's just and advice.
How you sure that ethereum will be going to POS?