In that case ...
@Robert Dalton
With the range of choice around the right 4500psi should be chosen, not just for capacity but for ergonomics
Aluminium cylinders have flat bottoms, you can stand them up on their ends, fibre cylinders have curved bottoms, they cannot be stood up but can be rolled in your shoulder giving smooth transitions
68/4500 is pretty much the default 4500 cylinder, but not necessarily the ideal for an individulal
It's fatter and longer than a standard aluminium 3000psi cylinder
At pressures up to 3000psi it will give you more shots just because of size (an extra 20ci of space)
Fibre cylinders are lighter then the equivalent aluminium cylinder, but a large fibre cylinder won't be lighter then a standard aluminium
(Unless it's ultralight such as 'safer', 'stako' etc)
48ci 4500psi cylinders have the same capacity as a normal 3000psi cylinder, but are short and stumpy and lighter. When 4500psi air is available you get 50% more shots
If you are after 68ci for that bit more physical capacity then a 48ci will get no more shots then your standard cylinder