With C-41 or E-6, there are a lot less variables in the processing, so when using labs, you'll find better, more consistent results. Kodak's Portra and Ektar lines are also "optimized" for scanning, so that will help also.
Indeed the Portra scan I got back from the same place looked much better to me, but I didn't know if that was due to some magic of the Rolleiflex lens or to the larger (6x6 vs 6x4.5) negative. Your comment makes it clearer.thanksgiuseppe
It's probably a combo of all those things. When you do a B&W film, they don't scan nearly as well (in my experience) and the choice of developer and process makes a huge difference in grain, contrast, and sharpness.