They have different re-test dates (see hpac website I think for dates, or use search button) Do the alu ones come in 4500psi? if not then thats a difference
Fibre wrapped are safer supposendly as if they explode the wrapping is meant to absorb some of the shock so they cause less damage than the others.
Im sure there are loads of other things but im half asleep and can't think
Sid