I have a brand new Falcon sitting on my driveway right now, having sailed one at the GC last year and been very impressed by it, so obviously my opinion is that of a man who chose the Falcon over other F16s...
The Falcon is a new hull shape, but in terms of build and set up it's very much an evolution of the Blade that Falcon Marine (formerly Vectorworks) have been building for a number of years. This shows in the design and layout of the various systems, which in my opinion, are now extremely well sorted as it comes from the factory.
For me, the extra weight ruled the Viper out, and that's not particularly a concern about boat speed or even ability to right: when the tide's out at our club*, the ramps up from the water are pretty long and steep, and the idea of dragging an unnecesary 20kg up those ramps single handed just doesn't appeal.
Beyond that, I don't think there are any great differences. The Viper uses an under-the-tramp downhaul system whereas the Falcon uses an internal downhaul system in the mast. Both are neat, but I had the internal system on my old Blade and I know it works well.
Paul
* Yes, I sail at an inland reservoir, but every few months London gets thirsty, or Thames Water screw something up, and we end up sailing on a puddle at the bottom of a 75ft deep reservoir.