Faster memory has little real world benefit though. The architecture of a CPU won't determine how power efficient it is - the design of the CPU will. 64bit has the advantage of needing fewer lines of machine code for certain tasks, but this will be negated by the fact that mobile apps are made for 32bit machines.
Actually, the architecture of the CPU is equally important when the reference designs are the same- both Qualcomm and Samsung are using the standard ARM designs for their big.LITTLE architecture. Samsung have the advantage of previously implementing such a design, and currently implementing a design that is manufactured on a newer process node.
The faster memory in the S6 also comes with a lower power consumption figure. In real world terms, it may have very little performance benefit, but it increases the headroom available to your system. I wouldn't discount it just yet.
My OnePlus One is the first non-Samsung Android phone I've had. I'm well aware of how performance increases when you flash a AOSP or AOKP ROM on them.
Fair enough, and agreed to an extent, but we don't know how much re-engineering has really been done on the TouchWiz platform to improve performance. Samsung have the closest vertical integration levels to Apple with this device- they manufacture almost every component in the phone. It's not entirely impossible that they've rewritten their entire OS to be on par with what an AOSP source could be. But, we'll know this soon.
Budget isn't so much a problem as it is a consideration. And it's one thing to be able to afford a flagship and another to be prepared to pay much more for the 128GB model.
128GB is at the extreme end, but 32GB and 64GB are pretty good options to have, too.