Density2 is the antialiased version of Density. It’s a little slicker sounding and less raw, so depending on your tones you might prefer it, or you might prefer the original. Like all the variations on Density and Drive, it’s got the highpass which feeds the distortion section, and the capacity to blend that with dry signal. That’s particularly useful if you’re using the highpass, because it lets you get some of the lows back, and it lets the distortion be applied to mid and high frequencies without getting muddied up with bass.
Again, this is the same thing as Density but with antialiasing to try and make the sound more fluid. You’ll probably end up with a preference for Density2 or the original, so just go with that.