Rick makes some pretty strong statements there, but doesn't say why. Perhaps I can shed a little light based on my experience. I, too, have used both tools.
I started on my empennage with a DRDT-2 and soon had my first visit from my EAA technical counselor. I knew he didn't have one of these tools. They were new at the time, and I was looking forward to seeing him "ooo" and "ahh" over this newfangled tool. Turns out, he didn't like it at all. I was shocked and disappointed, because I thought it was so cool. Turns out, I had the settings wrong and I was under-dimpling. He told me he had seen several DRDT's being used by newbies like me and every one of them produced under-dimpled dimples. This leaves large distorted or depressed areas, or "moon craters" around each rivet. Get the light reflecting off the surface of the skin and it really shows!
Later, I visited him at his shop and he demonstrated his C-frame for me. He whacked each dimple
twice with his hammer. He hit it hard, especially the second blow. It was like "whack, WHACK!", move to a new hole "whack, WHACK!" The first blow forms the dimple, and the second one really makes it crisp and sharp. He told me "don't be shy. You've gotta beat that aluminum into submission". I'll tell you, I was impressed. The skin around his dimples was dead flat and the light reflecting off the skins showed no distortion whatsoever. His finished rivets looked fabulous.
Since I didn't have a C-frame at home and I knew what my problem was, I set about to see if I could improve the dimples from the DRDT. I learned that if I adjusted the ram down some more, and locked it in place in a more extended position, it would work much better. I set it so the dies contact each other well before the arm is pulled all the way down. About 2/3 of the way down (this is before any metal part is in place for dimpling). It ends up taking much more force to pull the arm all the way down. You can actually see the big steel arms on the DRDT flex and bend apart a little bit under the force. And I pull down on each dimple twice with it, just like he did on the C-frame. The second pull, I even try to ram it down, or "whack it". From this point on, my dimples looked really good. You'd be hard pressed to see any difference.
Later on, I found an almost brand new C-frame for sale on eBay for half the price of a new one, and I couldn't resist. I bought it. So after using both, I can honestly say that if you adjust the DRDT like I described above and hit each dimple twice with it, you'll have very nice dimples and good-looking rivets. You'll have results you won't be ashamed of, and it's very hard to tell the difference. However, for the most discriminating builder, the C-frame does have a very slight edge, IMHO. You just can't beat those nice crisp hard-whacked dimples. That's my experience and my humble opinion. So what did I do?
I ended up using my DRDT to dimple all the ribs, stiffeners, flanges, underlying parts and structures, and so on, wherever I could use it. The bottom skins on my wings, and the belly skins on the fuselage. I still love the tool! But I used the C-frame for those dimples that are the most highly visible, such as the top skins on my wings, the fuel tanks, and the fuselage skins. Even though it was more work. Here's what my completed wings look like. No craters.
I hope this helps. The DRDT is an excellent tool and can produce excellent results! Like any tool, it has to be adjusted and used correctly to get the best results.