me personally i would rather go with the gas. But I wouldn't go with that brand. my buddy has a kwa m92r... the automatic beretta and they make a g18 that shoots around 320. only problem is it's like 100 more bucks, but it's accurate and reliable. He probally gets more gas like every two matches, but that's after reloading it with his speed loader like 3-4 times.
if you go with the gas option, they make a propane adapter so you can use regular propane. by regular I mean the kind from the grocery store, or a camp stove. All green gas is is propane anyhow, no matter what the chemical formula is on the bottle. So you can go spend like $17 on a huge ass tank and not have to worry about it for like 3-4 months.
No matter which one you go with I would invest in a hi-cap mag, the damn pistols blow through their 15-19 rounds in under a second. So get a 50+ round clip. If you get gas, get the 50, if you go AEP get the 100+.
The gas ones always scare the crap out of me when I'm getting shot at, and the AEP's sound like a regular airsoft gun... if sound is important to you. Anyhow, hope my little 2 cents helped.