Regarding how often you need to refill, it's not relevant if you have enough air to play the game you are playing.
You have to refill pods with paint and these days air queues are not as long as they used to be
You're 14 so either you have limited income from any job you can do, or you are funded by parents etc. Their funds may seem plentiful but you should also consider how far that goes
Are the fills available to you 4500psi? If not consider if it is worth paying over £100 more for a fibre cylinder then a steel/aluminium cylinder
The primary reason for various size and shapes of cylinder is for comfort and ergonomics. So with that in mind if spending about £150 on a cylinder then you should get it to fit you and not just the standard because everyone else does
Efficiency wise it is possible to run an x7 on a steel/aluminium cylinder at 3000psi which is hardly the most efficient. I have also used a 13ci cylinder on the x7 - that clearly didn't manage a lot of shots but it did the job
If funds are freely available and it won't affect how often you can play then a sensible answer is to buy the right size cylinder for you. If not so plentiful then use a 3000 psi cylinder
If you are concerned about the efficiency and you shoot a lot of paint then consider borrowing a cylinder at a site and firing over the practice range to see how much you can shoot on one fill
Then compare the shots you actually get with the shot calculator below - and adjust for other cylinder sizes and pressures. The figures will give an idea of the differences and information that can let you compare that to how much you shoot between games
http://www.scubatoys.com/paintball/paintballshots.asp
