As far as weight goes, they are the same as every other Classic shell (Seth's, Classics of course, Hot doggers) but I am not too sure if there is a weight difference between the FTi shells (Booter, High Five). But compared to a lot of the other boots i have tried, when they say half the weight they mean it. My Hot Doggers are the weight of one of my Nordica boots.
The flex is a 4 flex which is on the lower mid side of their range. I am 5' 9", 150 soaking wet and for me the 6 flex is perfect. The right balance of give but still strong enough to butter my skis and not slip out that much on big landings (I have afterbangs so I do anyway -_____-). For the flex it is more about finding what flex is good for you.
Compared to traditional i hear a 9 flex is the equivalent of a traditional 120. And each step down is roughly 10 percent. Roughly, because the flex kinda steps up. So the further you push your tongues of course the more difficult it is to flex so it's hard to give it an exact rating. So its kind of try them all ya know? Go to the shop and try flexing the DK then the Hot Doggers and then the Seth's, each is a step up
But all and all, for me the Hot Dogger is my ideal park boot and i am privileged that Full Tilts fit my feet. Hope this helps!