Choosing the right fly has been a topic in fly fishing literature since the dawn of all creation. Perhaps not that far back, but as far back as Dame Juliana Berners who penned a Treatyse of fysshynge wyth an Angle around 1496ce. In tactical or dynamic nymphing, fly choice is at the very core of the method. Matching the fly to the depth and speed of the current is now just as important as matching your fly to the local benthic macroinvertebrates that fish consume.
The Pellet Fly
Pellet flies a.k.a. perdigones fish best in pocket water situations when you will be making frequent close-in presentations. Pocket water requires that the fly gets into the strike zone instantly while being suspended under the rod tip. During these brief presentations, the fish only gets an instant in which to decide if the item is edible or not. The only way for a fish to determine the edibility of objects drifting in these fast currents is by taking it into its mouth and tasting or feeling what it might be. The fish either ingest or reject the offering in less than a second. I have seen this behavior while snorkeling in deeper pockets. Bright, attractive perdigones get the fishes attention quickly, and through the mechanics of tightline nymphing, hook themselves on our sharp hooks in the act of inspecting the fly.
The Dubbed Nymph
Dubbed bodied fly patterns fish best during extended range presentations. Extended range presentations are when a portion of the line interacts with the surface of the water or the fly drifts for longer distances. Longer drifts allow fish ample viewing time in which to inspect the fly both visually and through tactility. Another benefit I have discovered is dubbed bodied flies drift with a natural looking buoyancy when used in conjunction with a loop knot. Trout are also slower to reject the fly, which is essential during these extended drifts, thus allowing the angler ample response time to effect a solid hook-up. Conversely, Hard-bodied flies used in long range situations are rejected instantly resulting in less than satisfactory hook-ups. Some amount of dubbing is a useful feature to have a fly designed for extended range drifts.