the bigger the tank, the more shots...
But also the higher the pressure of the tank, the more shots.
4500psi 68ci = 50% more shots than 3000psi 68ci
3000*68 = 204,000 ----- 4500*45 = 202,500
so 4500psi 45ci contains almost the same amount of air when fully filled and will be noticably lighter and smaller.
But then you have to take into account that most sites only fill to 3000psi... how many shots are you going to make between fills??
My personal preference would go for the smaller bottle - just because it makes the marker setup lighter, smaller and easier to throw around.