People have some weird ideas on this thread...
You don't even need to measure frame stiffness, you can feel it, /especially/ when you are climbing out of the saddle. IMO climbing is one of the few occasions when frame stiffness /does/ matter, along with sprinting.
Frame deflection doesn't vary with power, it varies with torque, and that's much higher when you are climbing hard out of the saddle at 60rpm than when spinning in the saddle at 90rpm, applying the same muscular power at a higher torque and also your body weight to the pedals. Ok, so on a long climb and depending on your riding style you won't be spending the majority of your time out of the saddle, but for me at least it could be about 25% of the time, and that's when you really notice frame stiffness.
Of course this can be measured too and varies between frames, Tour magazine publish these figures on a regular basis.
Geometry of the Addict and Foil seem to be very similar, although the Addict seems to have a shallower head tube angle, which (depending on the fork rake) could in theory make the bike a little more stable but will also reduce responsiveness for descending, which may or may not be bad depending on how your brain is wired.
The Foil is supposed to be slightly stiffer to a degree that is noticeable. Personally I'd take that over a weight saving of 100g for climbing.