Mining - Few Questions

Hello everyone,

I have read many articles as well as many posts about mining lately.
I have grasped almost all the subtleties about mining but I still have some questions that I have not found a satisfactory answer, so I hope that the community can enlighten me.

1. Is it worth to invest in the RX470 rather than the RX480? (I heard that the MHs / Watt ratio is better on the RX470)
   
2. Is the VRAM different for each model? Is it detailed in the data sheet at the time of purchase?
   
3. If I understood correctly, the ETC is a branch that has preferred to "not evolve" towards the same technologies as the ETH.
   
4. (Poker) Do you have a (personal) estimate of the date on which the network will switch into PoS?
   

Thank you in advance for the answers,
Good evening.