For a phone called Pro it should have 6GB, future proofing is always a good thing. Don't forget what happened with the iPhone 6.
Future proofing is a great idea, that's why I'd like to wait for 5G capability. I've got a feeling that that will prove to be more of a necessity in the long run than 6GB (unless you need those to make 5G work better).
Of course right now I can't afford to upgrade to a pro, so this is all theoretical for me.