Each system has its pluses and minuses.
Spyderco Sharpmaker
It's simple, quick to setup, easy to use, can handle long blades, and very portable. However, it's limited to 30 and 40 degrees inclusive, so if your knife bevel is not set at one of those angles from the factory, you'll have to use it like a traditional flat stone or spend a really long time reprofiling. It starts at about $50 with 2 stones. Additional stones are available.
DMT Aligner
It's a clamp system, so it has many different angle settings, it's very precise, almost fool proof, and also very portable. All the stones are diamond, so it will work fast on super steels. However, it takes time to setup and it's cumbersome to use on long knives (over 4"). It starts at about $50 with 3 stones and a rod (for serrated edge). Additional stones are available.
Edge Pro Apex
It's a large clamp system with even more angle settings, so it's very precise and almost fool proof. The large size means it can handle long knives with no problem. However, it takes time to setup and it's not as portable as the DMT Aligner. It starts at about $150 with 2 stones. Additional stones are available.
Paper Wheel system
This is the least expensive option for power sharpening. Since it's a power tool, it's much quicker than the previous three. The configuration also allows you to handle long knives without any problem. However, it is not portable and it does need a separate work area. If you live in an apartment this is probably not for you. As with any power tool, it is also less forgiving of mistakes. I believe it starts at around $50 for two wheels, and you have to buy the bench grinder separately.