Basically, what
Jim Jam said. Basically, the more you pay for your PE marker, the smoother it's going to shoot, the lighter it's going to be, etc etc....
Etek (3, that's the only one I've had experience with) - Will shoot anything up to 4 star without a problem. Upgrade to a Cure 4 bolt and bore match nicely, it's somewhere on par with an Ego8/9. Fairly loud, but rock-hard reliable.
Ego - Think of it as an Etek with a few more extras - the board has a screen as standard, it's quite lightweight, more agressive milling, ready to shoot premium paint out of the box. Quieter than the Etek, also feels smoother in the hands. You can probably pick up a good used Ego for about £150 more than a brand new Etek.
CSL - Not held one, but I'd assume it is much the same as the Ego, only lighter and smoother again.
Geo - Has, as Jim Jam says, a board with a screen, shoots reliably and smoothly - may not be the most efficient of all the spooly's out there, but it is also reliable under pretty much any condition, unless you decide to play in the snow without relubing it. Same goes for all markers really. Considerably more maintence than the Etek/Ego/CSL - but it is a spool valve. Quieter, personally, I feel a much smoother shot.
Etha - I think this is an inline poppet valve based marker (Much like the Empire Axe) - not a spooly as Jim Jam says - but nonetheless, this is a much cheaper version of the Geo platform. Much simpler milling allows for the EMC kit to be put on the outside (if that's your thing) - also, it is fairly lightweight as markers go, and a fair bit of it is made from GRP, or Glass Reinforced Plastic. Awesome stuff. Also, means you can go sans gloves in the colder months. Has a weird "Baa-ping-ding" sound when fired, but that's the bolt spring resetting. Competely unnoticable when shooting more than one ball at a time. Also, the light on the back is bright enough to blind small animals.
Hope this helps!
