one question, does this device have expandable storage? if so then i would pick the 6gb of ram and 64gb of internal storage. you can always expand it by adding an sd card. but you can't expand memory unless you have the technical know-how.
If you are gamer, go for 6GB RAM. But if you are photographer and save much of photos and videos then go to 128GB memory.
I'd need to know more about the phone to answer. There have been cases where manufacturers have put a lot of RAM in as a marketing point but optimised the software so poorly that you can't use the extra RAM anyway. In fact my old phone with 2GB RAM would multitask better than my tablet with 3GB, so don't underestimate the importance of software in this. Personally I don't find 4GB on my Pixel 2 remotely limiting, but would regard 64GB even with expansion as the minimum.
There are several models covering brands like Asus, Sony, HTC, Samsung... The higher end devices usually come with 6GB/128GB config. But I'm limited to standard models, unless I'm willing to pay extra $100.
I will go with 6 GB/64 GB because at once Storage space can be sacrificed, but RAM can't be. The faster phone is the first choice.
If he's looking at different phones then which is faster may not be just a matter of RAM. In fact RAM (assuming it is used properly) will be more relevant for multitasking than for speed (i.e.once the app is loaded it won't make any difference - I'm assuming here that nobody is running an app that will not run properly in 4GB, because that would be insane ).
