I don't think you get any better quality scanning as a positive. I just haven't had any luck getting good B&W scans using Silverfast. The negatives always turn out with more contrast than I want and blown highlights. Even though it adds extra steps to the workflow, I get the best scans from that software on B&W scanning as a positive with the highlight and shadow sliders all the way out to the edges, and taking care of the levels myself. I still prefer this to the lower resolution scans that I was getting with the V500.
Silverfast handles color negatives just fine scanned normally.
I may try Vuescan soon, rather than keep this complicated workflow. I'd rather be able to get reasonable results straight from the scanner.
Examples:
Scanned using Siverfast, Negative, using the Plus-X setting for some Arista Premium 100 (Plus-X). This was the best I could do.
2014-11, Nikon S2, Nikkor 5cm f1.4, Arista Premium 100 - 20 by
Chris Bail, on Flickr
Same shot scanned as a positive and inverted in Photoshop. Levels adjusted and burned in the detail that was blown out in the first scan.
2014-11, Nikon S2, Nikkor 5cm f1.4, Arista Premium 100 - 20 b by
Chris Bail, on Flickr