Neither is better. It only depends on how you play or want to play. There are very few gas airsoft guns that are not either sniper rifles or handguns. Gas Blowbacks (GBBs), however, are by far the most popular gas guns. (I'll leave it to you to find out the difference between different gas guns.) GBBs are very fun and somewhat more realistic than most AEGs. They "blowback", or in other words, the slide jumps back, when fired and they have slight recoil. Most shoot less powerfully than an average AEG, although some are VERY powerful (and expensive). Gas guns also range in price from $80 to $250. Most GBBs are used as sidearms (you may hear differently from some "revolutionaries" on this forum).
AEGs are primarily rifles. Most airsoft players buy an AEG as their first real foray into airsofting. They are rather expensive however. Most are approx. $300 stock and brand new. They use batteries to fire as opposed to gas. They are generally more expensive to do upgrades, but most don't really need upgrades to be adequate skirmish guns.
Based on the above info, I'd reccommend an AEG for any beginning player. They are easier to take care of and use than gas guns. They are also more versatile and allow a beginning player to play at any pace or skill level he likes.
Really, you should just set your price range and find a gun you like the style of....Those are really the only serious concerns for picking out a first airsoft gun...Good luck.