For these two specific units ...
Wireless ML-L3 - good for distance and convenience (for example it's easier to be in the shot yourself), but bad as needs line of sight and is unreliable in strong sunlight.
Wired MC-DC2 - good for the fact that you can lock it with the camera on bulb for exposures longer than 30 seconds, but bad as can only be used as far as its cord of 1 meter.
Both can be used to fire the shutter without touching the camera so reducing camera shake, but beyond that they have their own specific purposes.
Rather than ask which is good from these two you would be better to ask why you want a remote shutter and what you will be using it for, then find one to fulfill that need. Personally I use two remotes, one infrared which has the capabilities of firing when the beam is broken and hence can be used to capture moving objects like animals. My second one is off of eBay and is a wired remote that has intervalometer capabilities, for example it can capture one frame every 30 seconds for 900 frames, cost was under $30.
