The amount of storage won't affect the speed - it has as much to do with speed as the color of the case the phone is in (until you start running out - then it affects it).
The difference between 6GB and 8GB of RAM? Unless you're running an app that requires a lot of RAM (like Firefox - which, if you keep opening tabs, can eventually need more RAM than you have, no matter how much you have - but I'm talking 35-50 tabs), the amount of RAM won't make any difference in speed. Yet. With the amount of RAM current apps need, even running 3 apps at once (say your banking app, your password app and Google Authenticator) won't come close to using 6GB. Running more apps? How many apps do you need to be switching between constantly? (I don't mean your game, your browser and your email app. You switch occasionally - I mean constantly, like getting to the login part of the banking app, needing the login and password, then needing the authenticator number - all before you can even check your balance.)
In the future? Who can say? Some games might start running huge amounts - 2 or 3MB. By 2025 we might see apps that are squeezed in 6MB. But are you going to keep your phone for 7 years? My 5 year old Note 3, with 3GB, has no problem running anything, including combinations, I run on it. 5 years old.
RAM isn't going to be the issue, storage is. 3GB phones still run fine, but 32GB of internal storage is no longer enough. (But you're talking about a 128GB phone, so why worry?)