If you've got the cash just get a 5000psi bottle, that way you can fill it anywhere.
If you get a 3000psi bottle you'll only be able to fill it where 3000psi fills are available and not all places provide this. With a 4500, or 5000 bottle you will be able to get it filled at places only providing 3000psi fills but of course you'll only get a 3000psi fill. That makes sense right?
So basically 5000psi = more air = more shots = better.
Oh and ensure its a hse or Pi bottle. DOT isn't legal in the UK.