I hear ya - It is hard when you have several factors to consider. Arizona snow tends to get pretty wet - Even though Arizona is a dryer climate the snow there tends to be wet because of fluctuating temperatures so to keep dry you will want to be looking for something with a higher mm rating.
For some good background on waterproofing check out the thread
Waterproofing/Breathabilityif you haven't already. This will give you some basic info on which to base the waterproofing/breathability decision on. A lot depends on where you ride and how much time you spend on your butt. Since you are new to this you may be on your butt quite a bit! In general for someplace damp I try to get people into something that rates at least 10,000mm and has the critical seams sealed - but proper fit trumps waterproofing any day.
The way you describe yourself I'm sure it is hard for you to find a good fit. The length of the Aperture pant doesn't sound so bad though. I have to admit that I see tons of people with pants that don't fit at all - I understand it is part of their 'look' but it makes me cringe. A pant that is too long will just get trashed at the bottom, the leg gaiters, zippers and bootlace hooks are totally useless and the articulation of the knees will be in the wrong place - basically there is no reason to spend the money if you are going to wear them that long. The cuff of the pant should really hit you at about the ankle bone (the big round ball on the inside and outside of your ankle) - just above or just below is ok.
It sounds to me like you may have a pant that works for you - especially if you like it over all - if it is just the feeling of them being high waters that bothers you then I would check them by attaching the boot lace hook to your boot and try sitting down - if you don't feel them pull uncomfortably then they are long enough. Keep in mind that snowboard pants fit differently than street wear pants and the key thing is that the snow cuffs don't ride up past the boot cuff when you sit or fall - if it does
then it is too short and we'll help you find something else.
Let me know if there is anything more I can help you with!