Have a number of Ahabs, several Konrads, vegetal, ebonite, and acrylic. Not much difference in performance, all required a bit of finessing to get to where I wanted them. But the final product is well worth the effort. The Ahabs have a faster flow and work out better if you are going for flex, in general. The Konrads work well with replacement No. 6 nibs -- be they from Goulet, Nemosine, Knox, or Conklin. Wouldn't try a TWSBI, as they are not standard-sized No. 6's. So do the Ahabs but --- I just prefer the feel of the Konrad.
Perhaps my favorite Konrad is a Noodler's Black Krakatoa Demonstrator with a Nemosine 0.8 italic nib. Keep it filled with Waterman Black and write daily with it. Two of my vegetal Konrads have R & K Scabiosa and Salix in them. Get a good, old-fashioned italic writing out to them, almost calligraphy grade. My acrylic has Montblanc Bourdeaux and the ebonite is filled with Noodler's Walnut.
Just one note: the ebonite and acrylic Konrads are about one-half inch longer than the older vegetal Konrads. Doesn't affect performance, just an interesting little note.
For Ahabs and Konrads, occasional maintenance is needed -- wash out, put a bit of silicon grease on the mechanism and o-rings, reassemble and fill. Goulet pens has the purest grease I have seen, works very well.