Like anything you are always going to get arguements for both sides. I own some of both, here's my opinion on the matter
Gas
Pro's - Nearly silent, extremely light trigger and bolt pull regardless of FPS, very few mechanical failures, ability to adjust FPS without replacing parts (on PCS bolt models), skirmishable right out of the box, nice stock externals
Con's - Higher starting cost ($450+), higher mag price, somewhat effected by temperature (not really an issue here either way), limited parts selection/availablility, can be finky at times
Spring
Pro's - Very low starting cost ($100+), parts can be found anywhere, more knowlege widely available, very lightweight guns, not really effected by temperatures, low mag cost
Con's - Need large amounts of upgrades to be effective ($300+), horrible bolt pull in high FPS setups, stiff trigger pull in high FPS setups, crap externals, more prone to failure (especially with higher FPS setups)
In my closet right now, I've got 2 Tanaka (gas), 1 Well/Ares (gas), 1 VSR (spring), 2 VSR clones (spring). I'll take any of the gas ones over any of the springs every time. I've used the Tanaka's in temps as low as 18° and they still outranged most guns on the field. If you are the type who is also concerned about the looks and feel of your gun, the gas wins by a mile there. The only spring sniper rifles I've held the felt NICE are the CA M24's. But they are expensive for spring rifles, and a royal PITA to get working WELL. The Tanaka's feel very nice out of the box and the Tanaka M700's will take real steel stocks designed for Remington M700 Short Action models with little or no modifications.