These two inks are similar in color, however, they each have distinct properties.  Even though the Rohrer & Klingner Scabiosa is an iron gall ink, I chose to compare these two shades for their color similarities.  Just keep that in mind when making a decision before ordering.  

Lamy F and 1.5 nibs on Rhodia paper.  Drying times are on the slow side even with the F nib.  Give them both a good 50-60 seconds to keep from smearing.  Both of these inks show their colors nicely with the F nib, however, the juicy grapeness comes through much stronger with a broader nib or brush pen.

Tomoe River paper shows the sheen and shade of the Barossa Grape and also has a smokiness about it while the iron gall Scabiosa darkens into a nice shade of what I would call eggplant.   

Robert Oster Barossa Grape on Rhodia paper using a ccolorless Tombow blending pen and a black belly roll pen for the wreath.

You really can't go wrong with either of these.  If I had to pick a favorite of these two purples, I would have to go with the Robert Oster Barossa Grape.  Although I feel it's less saturated, I love the smoky purple shade and the black sheen it can have.