The intermediates are good for a TRULY aggressive skier in the 120 to 170lb range.
the experts are for a heavier skier of similar skill in the 150 to 220lb range.
all they are doing is creating more elasticity for various amounts of force (knuckleing a booter, for instance), if you want to be tecnical, your kinetic energy, P=MV, is what you're considering here. So if you weigh 200lbs and case a 10 foot jump, you'll have the same amount of force as someone who weighs 150 casing a 50 foot jump, because they have to go faster, and therefor make up the weight required to have the same momentum.
These are NOT precise estimates, but just an example. These arent the weights that they are rated to, check with a boot fitter.
So if you're a light skier, you may not need a expert strap, just as a fat n00b wouldn't want a intermediate. If you're a fat pro, you're going to bottom out the straps, and they're basically not worth it at that point.
FYI, booster straps only kick in if you ajust your boots right, so see a boot fitter reguardless for the right fit.