A 3000psi steel/aluminium will do fine, a 4500psi fibre will give you more air when fills are available
The steel is still usable in speedball.
Questions that would direct to fibre are:
Are you of a physical size that would benefit from a specifically sized cylinder? (EG the distance from your shoulder to shooting hand)
And you a small / young person who would notice a weight difference?
Are you just learning speedball in your training? (In which case 3000psi will still do for starters,
and it won't get damaged from practicing diving, sliding etc)
Are you joining a specific team? (In which case discuss cylinders and makes with them - there could be sponsorship implications)
The lighter the better, but if you just start playing its ok to have 3000psi, since you never shoot away this in 1 or even a few games, ofcourse it depends a the marker you're using, how much you exactly shoot.