New guns depreciate in value rapidly once taken out of the box and used. Much like cars driven off the forecourt.
Always look second hand for good value on your markers, however for a new out of the box marker that shoots great, I can't fault the Drone 2. For a retail value of £500.00 it performs the same as the Geo 3 +.
There are always additional 'bolt ons'/ upgrades that you can get for it too.
At the moment though, with new gear and products being released in Chantilly with the Mills world cup, and the NXL world cup too, it might be worth waiting a tad as there will be loads of people selling off their markers to afford the brand new shiny stuff, so great opportunity to get an upgrade.
Re. what to get, it really does depend on a budget.
Geo 3.5 seem to be going for around £600+
Geo 3.1 around £450
Geo 3 about £300
other manufacturers I am not so sure on values, but I know that Dye guns depreciate rapidly.
(someone correct me if I am wrong on these)