The Viper is a fun ship to fly. The engine sounds like a big block V8, and the small profile of the ship almost feels like you're flying an A-Wing. It's still a little big to fit that small interceptor role, although the f63 might be Elite's A-Wing. As far as firepower goes, the Cobra can fit bigger weapons, and the DBS has better hard point placement as well as bigger weapons. Also, you have more freedom of travel with both the Cobra and DBS, at the expensive of a slightly bigger target. The Viper is just a harder target to hit, but I found that anytime I relaunch a Viper, I end up missing my Cobra. Maybe it is just the way I fly, but I have the same level of success flying a Cobra than I do flying a Viper. I guess I just like being able to boost to 430 anytime I want, and then make a 20 ly jump somewhere else. I know it is subjective, but I feel that if the Viper is an 8/10 in terms of combat capabilities, then the Cobra and DBS are about a 7.8/10, but by sacrificing something trivial that can be worked around with piloting, you gain utility, jump range, and more fire power.
To be honest though, I'm currently I'm in a Vulture, and it has sort have been my end game ship for now. I know it is sort of an OP ship for PVE for the price, and to some people that means it is boring. I just haven't felt the need to go back to my Viper, even for PVP. The Shields on a Vulture are in the range of a Python if you stack boosters and an A5 Generator. The only thing I miss is the jump range and not having multiple module slots of the same class. With a Cobra you can fit 3 shield cell banks and leave two disabled. With the Vulture, you have to either roll with a class 4 bank, or roll with a weird shield stacking layout of 4, 2, 2, 1 or something like that. It is just a little awkward, but the shield tanking makes up for it.
Anyways, I know this thread wasn't about the Vulture haha. The Viper is still a good ship and hasn't been completely replaced. In PVP I would probably still fly it if my group already had Cobras. Hopefully there will be future updates that give the Viper a new role compared to the Vulture, maybe with CQC.
TL;DR: I prefer the Cobra, but the Viper was fun to fly, and it's engine screamed.