The Cobra costs more, and has higher classed equipment which costs more. The higher classed thrusters help it maneuver about the same as a viper.
Is the viper a good endgame ship? No.
However if you want something fun to go bounty hunting in, that is cheap to outfit, and replace there isn't a better ship, I think it costs about 2 million to fully deck out a viper, where as the cobra is much more expensive.
Personally I take the versatility of the Cobra over the Viper any day, but there are many bounty hunters out there who think the viper is the bees knees.
The Cobra suffers incredibly in turns. It drifts hard in medium to high speed turns. It also takes quite a long time when initiating the turn to eventually catch up and start to turn to it's best ability. This is an issue the Viper really doesn't experience. I've decked out both the Viper and Cobra, experimenting with different builds and I've stuck with the Viper for combat.
The Viper is definitely a better ship for combat. The Cobra is able to do other things better and is a perfect ship for the less than wealthy beginning trader.