I'll choose 570. Possibility of 4gb ram issue is very small, PoS conversion will be completed before that happens.
4GB will be there for about 3 years. During this time, can still very much change. Even when you will not be able to mine with GPU Ethereum will be other coins. There is now a bitcoin gold for example. Sure there will be others. In any case, you can always sell your GPU gamers. In the case when you buy an asic you can only throw away when it gets old.