There are too many variables in gas rifles. Unless you are using compressed air, there is nothing more consistent than Spring power. Gas rifles are realistic, but not very practical in most situation.
Propane and 134a adapters require lubrication when filling, this keeps your seals from drying out and leaking. However, it also sprays a small amount of lubrication on your BB's, this can be a pretty big downside. I've noticed this happens on just about every GBB I've had, and with my gas M700. Its better to use grittier BB's with gas guns for this reason, I feel it bites the bucking a little more as it goes passed it.
Higher power gas rifles in lower pressure areas also produce a nice sized plume of gas that leaves the barrel, often enough you lose sight of your round and cannot tell when its lands. Obviously this inhibits you from making correct adjustments for the next one, but the use of a suppressor will keep that plume under control. But even a suppressor can create turbulence of air depleting accuracy, that is a very minute factor though.
Another thing I don't care about gas rifles, is that there are a lot of moving parts that are not replaceable, there are small pins which are easy to lose and never in stock anywhere. This case goes with GBB's too, pretty much relying on you to fabricate the part if you don't want to have a gun out of commission for months. Every part on a spring rifle is obtainable, aside from accessories to aftermarket parts, you can get just about anything that breaks.
Typically, you can have a spring rifle field stripped in 2-3 screws. This includes the outer barrel removed, hopup completely dismantled, and the bolt & handle apart. That's just a total of taking apart 4-5 screws depending on your model, to have literally everything apart.
Last but not least, unless you live somewhere that is stays over 50 degrees all year, you may have a gas rifle treat you well. But if you live on the east side, forget about it, save money and buy spring. In freezing temperatures, any gas rifle is useless if more than five shots needs to be taken. Condensing gas will freeze in your barrel, and there is no way around it. This can even happen with spring rifles when cold enough. It can harden your bucking making it shoot like a complete pile of shit. I've had this experience at least once a year for the passed 5 years with my rifles, spring and gas.
Pro's of gas rifles.
-Easy to cycle
-Easy hop adjustment
-Comes shooting hot
-Longer barrels stock than spring
Cons of gas rifles.
-Frequent Hop adjustment
-Generally Inconsistent FPS readings
-Need to pay for gas
-More Moving Parts
Spring or Bust.