1. The Viper is bigger than the Talon. However, the Viper is not a brick, by any means. Nor is the Talon a "small knife." I would say the Talon is the more pocketable of the two, although that being said, it's a fairly meaningless distinction because I'm carrying my Viper today, and I'm wearing dress pants.
2. Both knives have really sweet pulls, that feel perfect for their respective frames, and have very nice walk and talk. The Viper's pull is stronger than the Talon's, but it's not a nail breaker, nor is the Talon weak.
3. Both knives have very nice aesthetics and are eye-catching. They also both have a unique feel in the hand that most people love or don't mind, and a few hate. The swayback is not everyone's cup of tea, and the Talon's eureka pattern echoes this. I personally find the Talon just a touch better in the hand, but this is splitting hairs, and is also highly dependent upon the individual.
4. Basically, they're both big-ish single blade wharncliffe knives, which is a highly specific type of knife that might be perfect for you, or might be unsuitable, depending on how you use a knife. They both do the single-blade-wharncliffe thing very well, and the only difference between them is one of aesthetics -which look you prefer, or which is available. In practical terms, right now this means the Viper is the knife for you, because they can still be found fairly easily since there was a recent run of them made. The Talon, especially in its wharncliffe variant, is effectively unobtainable at the moment. There have been rumors that GEC might do another run on the 92 frame later this year, but if they do, it's anyone's guess if they'll do single wharncliffe again, or something else.