For the most part spring and gas powered airsoft sniper rifles both offer the same performance. However, there are definitely some pretty big differences between the 2 types of guns that you should know about. For example, you will never need any additional power source in order to fire a spring airsoft sniper rifle, but you will always need to purchase gas in order to fire a gas powered airsoft sniper rifle.
Also, another difference between spring and gas powered airsoft rifles is the fact that gas airsoft sniper rifles can sometimes be affected by temperature (for example if it is really cold out). Spring airsoft rifles always perform exactly the same no matter what the temperature is, since they are powered by a spring rather than gas. The discharge rate of gas is generally slower when it is cold, which means that your gun may shoot a bit slower if it is really cold outside.
If you are looking to save money, then it is likely that you will want to go with a spring airsoft sniper rifle, since the gas powered ones tend to cost significantly more. In fact, gas airsoft rifles almost always cost about $50-$100 more than spring airsoft rifles. And again, you will need to keep buying gas for a gas powered airsoft gun, which makes them all the more expensive. With a spring airsoft sniper rifle the only thing that you will ever have to buy is the airsoft BBs that it shoots.
As far as looks go, both versions (spring and gas) of the sniper rifles look pretty much identical, other than the magazines. The reason that the magazines dont look the same is because the gas airsoft rifles actually store their gas in the magazine, while the magazines for the spring airsoft rifles store nothing other than the airsoft BBs.
Literally every spring airsoft sniper rifle is a single shot (usually bolt action) gun, which also holds true for most of the gas powered airsoft sniper rifles as well. However, some of the gas powered airsoft sniper rifles are semi-automatic, which is a huge benefit that spring powered airsoft guns will never be able to offer (since you have to cock back a spring for each shot).
When it comes to actual performance, generally gas powered airsoft sniper rifles shoot at a slightly higher velocity VS the spring powered ones. On average gas airsoft rifles shoot about 50-100 FPS faster than spring airsoft rifles. Also, it is much easier to cock a gas powered rifle since there is no heavy spring to pull back.
Accuracy is exactly the same between spring and gas powered airsoft sniper rifles. However, in some cases you may want to use heavier airsoft BBs for gas powered airsoft guns, due to the fact that they usually shoot a littler faster than spring airsoft guns (and the faster your airsoft gun shoots, the heavier the BB that you will have to use in order to maintain consistent accuracy).
When it comes time to decide whether to go with a gas powered or spring powered airsoft sniper rifle, just remember that each one has its strengths and weaknesses. If you want a semi-automatic airsoft sniper rifle, then you will have to go with gas powered one. If you want to save money, and never want to worry about buying gas, then go with a spring powered sniper rifle.