I used to run 391's and I recently traded "down" to the c-4s. I didn't like the 391's at all. My issue with the 391's was the higher end and "presence" responce. IMO they suck in that regard. Unless you were up close the sound was not at all what I was looking for and even up close they didn't have a very realistic sound to my ears although it was better. The 393's are a completely different story, they are a far better cap to my ears, but they don't come around very often. The C-4s have a little bigger presence boost that brightens things up a little more, and also come with a second set of omni caps which I use a lot. The AKG's are by far a better built mic. The c-4s don't have quite the engineering built into them, but the sound is far better to my ears, and the cost is substantially better.
Just my .02
Matt