Maybe 15 years ago I bought a Viper. I had broken my left wrist and thought I'd try it. Used it for about a decade....until I decided my lighter Zaveral paddles were not going to break with frequent use on rocky streams. I DO feel like it's a good design and did take some pressure off my wrist and elbow? Pure speculation though, may have just been a "Placebo effect" because of the radical style?
Anyway, I gave the paddle to my friend's wife 2 or 3 years ago (mine prefers a kayak paddle) and she loves it. So does he, so he uses it every day his wife ISN'T using it! Last year, I was at Rutabaga's and I bought a new one thinking a longer one may be better for kneeling in the boat I'm paddling now? Unfortunately, it was a bit too long for my taste, so I sold it. Not because I disliked the paddle, though.
Never used a Sawyer Manta, but I have paddled with friend's Sawyers and they seem well balanced and well made. I think I would actually PREFER the Manta because of the T-grip and, having used the Viper for over a decade feel a little extra $$ for a different grip would be worthwhile? I do like those. Because, if I had one complaint about the Viper, I felt the grip was a mite large?
Hope that helps?