you will get more shots from a 4500psi tank as it holds air at a greater pressure, therefore holding more
stako bottles are great if you can afford one, if not then pmi/evil/crossfire/dye throttle are all good
4500psi will hold more than a 3000 if the site fills over 3000psi. Fibre wrapped is also lighter, the lightest bottle out there is the stako, but they are expensive and only last 5 years.