This is really an odd question......It's multi-variable calculus in an alpine environment, and the reason why you're not getting a straight answer, Michael.
Actually it's a good question IMHO, as I frequently refer to exposure in snow preservation comments and assume that most skiers know what I mean. Michael is perhaps not getting the simple answer he may have wanted, but he's getting more nuanced answers which are hopefully informative.
The best aspect is completely subject to weather, temp, wind, etc... on a given day.
We've all had some days where the north faces are refrozen and sun softened exposures had the best skiing. But over the course of a whole season (and particularly the late season) there will be far more days when north is preferred.
you'll still ski south'ish facing aspects later in the season at lower altitude than a place like JH because there's deeper snowpack... Now you have add continental vs. maritime snowpack in the equation. Then there's snow stability and safety to the two aforementioned if you're considering doing any ski touring.
While JH snowpack is not as deep as PNW snowpack, I'm fairly sure that in a normal season it remains covered to its early April close. Spring closure of the lower faces at Jackson in March is more likely due to refrozen conditions or wet snow instability considered dangerous by patrol. These issues within most ski area boundaries at more north-facing Alta/Snowbird are infrequent until mid-May or so.
My understanding (from the ISSW conference I attended 2 years ago) is that wet snow instability is not yet well understood. Thus A-Basin's conservatism about closing Pali early to mid-May since the late May 2005 accident. I'm fairly sure than when snowpacks get down to 2-3 feet and become isothermic they are more unstable. So that would contribute to the rationale for the PMW late season snowpack being safer, particularly in the backcountry.