KYB shocks are pretty good shocks for the money. Not only do they supply OE shocks to Mitsubishi, they do so for Infinity, Nissan, Lexus, Mazda and Subaru as well. They have to be pretty reputable for these makes to pick them up as standard equipment for a lot of their models.

Bilsteins are a lot more expensive due to a number of reasons. Bilsteins offer shocks that have more aggressive damper settings that work great with a lot of shorter, more aggressive springs used in a lot of racing applications (on both streetable and track-only rides, I might add), as well as touring-type shocks that provide a smooth comfortable ride that may be similar to the original shocks, but with some of the more sporty characteristics of the more aggressive line of shocks.

Also Bilsteins will net you a (generally) longer shock life with consistent performance allthroughout.

They are quite expensive, but if you have the cash, they will most certainly live up to their reputation.

Mixing and matching hydraulic and gas shocks on a car is just asking for trouble. It's like having different sets of tires with different attitudes and levels of grip. At the very least, it'll make your ride feel weird, and at the very worst, it could be potential road hazard and an accident waiting to happen.

Hope this helps!