Just confirm if it is steel or aluminium
Most steelies are aluminium, and are a similar weight to the large fibre cylinders.
Hence if you buy a large fibre then you have not saved weight
Buy an equivalent fibre then it will be lighter then aluminium and much lighter then steel
There are some weights here for comparison
http://p8ntballer-forums.com/threads/bottle-weights.158899/#post-1444503
Get the right shape & size for you and the ergonomics are right
But note that it's not purely weight, but balance that is the true factor. If you get the right balance then you don't feel the weight.
Costwise a new fibre is approx £150, specialized ones with specific regulators or ultra light designs will be £200+
Second hand fibres usually sell for £80 irrespective of age
But check the dates, test status etc
For a guide I value fibre cylinders based on a rough lifetime cost of £200 including 2 tests (handed over) at £25 per test
Divide £200 by 180 and you have the 'value' by month, multiply by the number of months to its final date, then deduct £25 for each remaining 5 years to cover any future tests you need to pay for