Greetings:
Some of what you mention needs some clarification. Most springer rifles are not that good. The better bolt action sniper rifles (meaning
not the Super 9), are actually spring powered, and dang good. The same wide variance in quality and performance is available in shotguns. The Hardball II, and other 'inexpensive' shotguns like that are not worth it unless you are only playing against people using low end springer pistols and rifles.
The big difference in the higher end shotguns, in gas vs. springers, comes down to how you are using it, and where you are using it. The spring powered ones like the M3 Super 90, because of the much higher fps, can be great at longer ranges, but because they take some effort to

, are somewhat cumbersome in CQB. The smaller gas powered shotguns,

much faster and easier, giving you a faster second shot reaction time in CQB, but lack the range, accuracy, and power for extended distances (over 50 feet). There is also the issue of keeping track of the shells. The gas powered ones eject the previous shell when you

it.. meaning you have to find it, and hope you don't step on it. The Tokyo Marui ones, you have to manually remove the shell to load a new one in, but you get to shoot 10 times per shell.
Hopes that answers some of the questions.
Dughal