Bit of a necro post but since the issue was not answered in-house I figured I'd take a swing at it.
I have personal experience with HPA and CO2. HPA holds much less by volume since its compressed gas and not a liquid like CO2. So, 12g CO2 powerlets hold a decent amount of shots in something like the Cold Shot. On the other hand CO2 still fluctuates as it boils off, its temperature can effect HU buckings, its still affected by the weather, still has a cool-off effect, and causes issues with pretty much everything if it gets in your internals as a liquid. As a liquid, CO2 boils off as it leaves the bottle and provides a very good source of power if you are just trying to power a gun. CO2 is most common amongst classic users since you can get a fill in most sporting good stores and its cheap. I use CO2 mostly. Using CO2 with a remote line or regulated air rig pretty much nullifies any concern of it as a liquid entering your gun. There's just too much room for it not to boil off. The cool-off effect is not much of an issue either due to the volume used with CO2 tanks compared to gas-in magazines.
For snipers, and even more so, snipers with deep pockets, HPA presents itself as a much more desirable power source if lugging a bottle isn't an issue. I personally use an old camelbac or chest rig and pack, whichever is easier. It's much more consistent since it is just pressurized gas. It leaves the bottle pretty much the same way it entered. Its not effected by the weather, and it is not going to effect HU do to its temperature. Volume isn't really an issue with snipers since shooting a bunch is out of the question. Its common to see HPA increase performance besides consistency. In general, ROF increases, operation is smoother, and there is absolutely no cool-off effect. Filling HPA vs. CO2 is a little harder. You'll need a paintball store or scuba shop to help. It costs just the same as CO2. If you are really a lone wolf or just out in the sticks you can buy a SCUBA tank and use a HPA yoke to re-fill your own tanks. Re-filling HPA is much easier than CO2. That venture could be $200 but its not like you haven't spent that on some of your other gear.
HPA is the obvious choice right? The issue remaining with both power sources is that you'll still need a $100+ air rig, a bottle and (with beating drums) you have to mod your gun to use either one. Find someone who knows or learn how to drill and tap. Tube and air fittings can be bought all over the place. McMasterCarr.com is a great place if you're out in the sticks. Once you are all hooked up and have bought your new gear the only thing you should worry about is a little silicone oil. I use 100% silicone light weight RC shock oil (Preferably 10W or 10 weight).
But what gear should I get?
You can buy air rigs pre-made or used from members on the classic airsoft forum. You can also try making one yourself. Their are guides for that too on said forums. Whether you choice CO2 or HPA I think you'll be happy over green gas or propane. Think of this as buying a cylinder set or a really nice scope. HPA bottles don't have to cost a lot for snipers unless they want them light.
Bottles come in various shapes, sizes, volumes, materials and pressure ratings. Carbon fiber wrapped carry the most pressure, weigh the least and they are the most expensive. Aluminum is probably the next choice since the only other option besides that is steel. Both Aluminum and Steel tanks hold up to 3000PSI where carbon fiber wrapped will hold up to 4500PSI, meaning you have more shots. From what I've seen Crossfire makes the best tanks for our purposes. They are inexpensive, quality tanks that have key aspects of performance that are desirable. They come in different materials, PSI pressures and output, and volumes.
Are low pressure output bottles better for us?
Unregulated HPA is about 950PSI and all tanks restrict this but some restrict it to as low as 450PSI. This is good because your regulator will not have to work as hard. You probably will not find a reason to go over 200PSI, using the alternative of a 450PSI tank means that your reg will have to bottleneck 850PSI down to a manageable pressure. This requires it to work harder and larger fluctuations in gas will exist.
I use two regs on my air rig. The a Palmer's Pursuit reg that screws on to the bottle (CO2 and HPA use the same threads). I regulate the pressure out of the bottle to 200PSI and use a secondary mini reg to below 150PSI. Using multiple steps in regulating your PSI will help with consistency and provide smoother air flow. I also have a coiled section to help with any hangups of brush and I have yet to have a problem.