Stiffer boots (when they fit you) give you more responsiveness... if you are in control of your body... than that means more control of the board. However, where you are a novice (often "intermediates" as well) you tend to flail your limbs everywhere... and in that case the extra responsiveness may not be what you are looking for. Fit is always more important than flex... once you have a good fit, you can play with flex to your preferences (which as a novice will change over time).
Now, I will add in the obligatory, but very important "You should try other brands if your heel is lifting in this brand's boots..." a heel kit is just a band-aid to the bigger problem that the boot isn't shaped like your foot. If you foot is moving around in your boot (i.e. heel lift) that makes it much harder to control the board (imagine if the steering wheel of your car was made out of cardboard... you can see how that floppiness would make it hard to carefully steer the car through an obstacle course. A stiffer boot that fits will is very comfortable. If it feels like it is digging into your foot... the boot doesn't fit well. If the boot feels like there are large empty spaces... the boot doesn't fit well. If you heels lift when you bend your knees into a squat... the boot doesn't fit well.
Snowboard boots are made for snowboarding... that involves going high speeds on a piece of wood attached to your feet, hence it needs to have some stiffness to support you. Snowboard boots are not meant for strutting around at the base of the village pretending to be a snowboarder. I would recommend you buy boots that are good for snowboarding and less based on appearance (color) and "walkability" like Lil Wayne here...
The crazy thing is all of that shit he's wearing will come off, and it will be at the same time as that good looking chick that's walking next to him.