For the majority of riders, standard insulation is all you need. It's designed to handle typical winter resort conditions, keeping your feet comfortable in the 15°F to 32°F range—which covers most days at most mountains. Standard insulation strikes a nice balance: warm enough for cold mornings but breathable enough that your feet won't overheat and sweat when the sun comes out or you're hiking the park.
Heavy insulation is really only necessary if you regularly ride in sub-zero temps or you're someone who always struggles with cold feet due to poor circulation. Going overboard with insulation can actually backfire—sweaty feet get colder than dry feet, and overly warm boots can make that happen fast.
One tip: if you do hit an unusually cold day, you can always add boot heaters to a standard-insulation boot. That's a more flexible setup than locking yourself into a heavily insulated boot that might feel like a sauna on normal days.