ohms?

With a 4 ohm dvc you have the option of wiring it in parrell or series...which will create differnt loads that the amp will see....for instance a dual 4 ohm voice coil can either be wired in parrell for a 2 ohm load or in series for a 8 ohm load.....so you have more options with a dvc compared to a singe voice coil....

is 2 ohm or 4 ohm better?

well that depends on your amp if  your amp is 2 ohm stable....if not then a 2 ohm load would not be best...

2 ohms is no better or worse than 4 or 8 or anything else.  It depends on the quality of the speaker and the quality of the amp.  The purpose of knowing the ohm rating of the speaker (and the purpose of being able to wire systems for different ohm loads) is to match the speaker to the proper amplifier.  The only reason to run low ohms is to squeeze more current out of the amp - which makes it louder, noisier and hotter.  Higher ohm loads means the amp will run cooler and with less noise.  It will also last much longer.  The best sounding most high quality system I ever built - an SQ award winning system, by the way - used a set of subs wired to load the amp at 16 ohms.  It all depends on what you are trying to accomplish.

For most people who just want a good sounding system that will last a long time and not fry, run 4 ohm speakers.