a biger psi bottle holds more air, so you get more shots per fill.
preset means that it has fixed output pressure either High presure (HP=850psi) or low pressure (LP=450psi)
if you can afford it, go for a 1.1l fibrewrapped bottle in 4500/5000 psi size.
Just paintball are currently doing these with reg for £99.95 you just need to know what input pressure your marker needs