My opinion about the relative snow conditions at Alta and Snowbird is based upon my experience, which is mostly in March and disproportionately in warm weather. This is the specific condition where Snowbird does better. The difference is not huge, 10-20% more of Snowbird's terrain will be packed powder vs. spring conditions at Alta, but I've skied the areas on consecutive days in several seasons and found the difference to be consistent.
I already commented that with similar weather in January this year, conditions were exactly the same. And early season until the base gets up to 5 feet or so, no question Alta is better. So rfarren's observations may be correct depending upon the timing and actual weather of his Utah trips.
I know some of it is tram access to the upper mountain with a downloading option, but it seems eminently logical to me that Snowbird is the area that stays open to Memorial Day or later when there is not enough local demand to support more than one area being open. I'm fairly sure both are on Forest Service land, so I don't see why permitting would be different.
I still have a hard time understanding why both Alta and Snowbird devotees are so polarized. I may fall in the latter camp, but evaluated individually surely both are top 5 in North America if you're above intermediate level. It seems particularly counterproductive for Utah locals who can take advantage of the time-of-season variation to be as exclusive as some of them are. The AltaBird pass doesn't make much sense for a daily visitor given the scale of both areas, but for Utah season pass locals it's a no-brainer IMHO. Even with my time at the Iron Blosam and expressed preference for Snowbird my lifetime day count is 52 at Snowbird and 21 at Alta. Vertical difference is well over 3-1 due to the legacy of Alta's tedious lift system before the new Collins chair.
I'd be interested in BobbyD's observations, since he actually uses his AltaBird pass at both places on the same day periodically.