For function they are no different - they both feed air into the gun
For weight there is not a great difference, two equivalent 0.8litre bottles are different at 1000g and 1600g, a saving of about 1/3rd. It's lighter but unless your a skinny, slight weakling 600grams isn't a lot.
In the past people haven't gone for the equivalent but the more common, when going fibre they then go bigger and still claim that the standard aluminium is heavy even though they now have a bottle of the same weight or heavier
(Balance is more important than weight)
Unless you're getting an ultralight fibre then weight is not a genuine argument
VFM an aluminium is about £30, whereas a fibre is about £150
The aluminium is then good for 5 years, or 10 if the site accepts it,(£6 or £3 per year) then if you retest it it could last forever but isn't really worth the effort as you can buy new for not much more
The fibre costs £30 a year if you count the first 5 years, but then you retest at the 5 and 10 year point making a lifetime 15 year cost of about £200 - £13.33 per year
Fibres can hold more air, but only if you get a 4500psi fill and then get 50% more air. Unless only 3000psi fills are available and you still get the same amount of air
You can still get a bigger fibre to get more air for your money, but with aluminium you can buy two and get double the air and not worry about higher pressure fills
If you can shoot enough paint for a game then you have enough air
Aluminiums can happily be battered about, fibres need to be protected - they won't fail but scratches and bumps will affect the fibre weave and one day a site will refuse to fill rendering the cylinder a paperweight. So a cover is essential to protect it - not lots but more £
A disadvantage of aluminium is the curved base, but you can spend a bit more about £40 and get an aluminium with a curved base
This gives the advantage of rolling it in your shoulder
The real advantage of fibre is the range of shapes and sizes you can pick from to meet your needs, physically fit you and give the right balance
http://p8ntballer-forums.com/threads/bottle-weights.158899/