The MP20 reticle offers the advantages of a milldot for ranging and for hold overs without the obstruction of the view. IE this reticle uses lines instead of the larger dots which can obstruct the view. Also it gives you new crosshairs at various points.
The advantage of the metric is that the distance between the big lines is the same as that with a mildot system. However with metric it's 10 clicks for the distance and 5 clicks to the half lines. So adjustments are much easier. Less math, more exact. At least that's what I've been reading. I don't have one yet and I'm far from an expert on the subject.