Well I personaly would go with gas just because there is less chance of it breaking under to much stress.
Spring
With spring you have to make sure you have all the right parts, and tune it just right. You dont want it to go over 500fps. To tell you the truth keep it under 500fps if its shooting 470+ keep it their. You want it to have a better life span, and you dont want to have to use to much strenght to pull the bolt back.
Gas
With gas it will last longer if taken care of. Most of the tanaka guns you can adjust your fps just like a hop up. Problem is freezing winter months will make your gun inaffective, unless you keep your gas warm. Drastic changes in weather will alter your consistancy and you may have to ajust your hop up, and scope. You wont have to make any upgrade with a good tanaka rifle. It will shoot 500+fps out of the box with green gas.
I just bought a Tanka AICS and I love it. I still need to ajust the hop up alittle more. I had to put about a hundred round through it to break the hop up in properly. I am thinking I might by a spring rifle in the future for the cold winter months up north. A CA24 or a Sun project M40A1