In a good dry warm long Summer, climate,Marseilles black vs is very close in quality to Madeira Black,just like Mike ,graded them.
In short cold Summer climates,Marseilles black vs get ripe ,an abundance of fruits ,that are of medium quality compared to a couple of ripe fruits of higher quality,Madeira black will manage to ripe in said climate (This is what happened to me here in NJ,this year).
I also had super tasty flavorful fruits from Marseilles black vs,in some years,last year was 2010,and I remember a very good year in 1997.
In 1997,i had super tasty fruits that lasted from August 1 to December 15.
In December I had some very very tasty fruits ,on a tree that already shed it,s leaves!.
Of course it did not happened again to date.
A good long hot Summer is the most important,to get good fruits from both.