Good plans on following Mikes tips.
But do put it into a little context for yourself.
Where do you play and what fills to they have?
e.g. If the site still uses Co2 then your own air cylinder wont help without the added expense of getting offsite fills (and giving the ability to run your own marker that does not like CO2)
If they run air then you can still borrow cylinder when you play if you save for a marker first.
The advantages of a fibre cylinder is choice of sizes, less weight, more air due to size capacity (unless you go for small due to your requirements), more air due to pressure capacity (provided the site/event offer 4500psi)
The disadvantage is cost. About £150 from new and committing to a 15 year lifespan and requireing 2 re-tests to get that lifespan. (Minimum £200 life cost)
A 3000psi aluminium cylinder is not a bad buy, it saves money allowing you to keep it for the marker and paint.
At about £30 you can discard it after 5 years, but if you get it retested then it can last forever as there is no lifetime limit provided it remains in good condition and passess tests.
I would advise on your next purchase being the marker. You may negotiate a deal for marker and cylinder together.
If its going to take time to save for an Etha then that is another reason to hang onto your money and shorten the wait.
You can happily run an Etha on a 3000psi aluminium cylinder. It will be vastly more efficient then the rentals you are using.
An fibre cylinder will give you more options, but it costs more.
