I should probably add that there are risks associated with both Squaw and Mammoth.
As I said before, Squaw can be all rain, which can ruin a good snowpack. 
 Mammoth rarely rains up high, and it is very consistent all winter.  The upper mountain gets good windbuff, which can mean good conditions weeks after a storm.  The upper mountain really does tend to ski well all winter, and it rarely gets slushy until spring.  Having said that, if the upper stuff does get slushy in January, you can be fucked for weeks.  The conditions that keep the snow wintery up high can keep slushed up sections icy for weeks once the normal weather returns.  It happens occasionally, although not too often.  With Squaw, you have 360 degrees of exposure, so even in January you can make good slush turns on Broken Arrow, and good windbuff turns on the north facing KT alternates.  If the whole mountain at Squaw get slushy, then freezes, Broken Arrow will almost always slush up.  Mammoth is not that way, at least until spring.  As I said before, if the top of Mammoth gets slushy in January you are usually fucked until there is another storm.  Squaw's exposure makes that less of a problem, as there is both south and north facing quality terrain.
As long as the top stays wintery at Mammoth, which it usually does until spring, the wind will continue to smooth things up.  If you follow the wind, Mammoth can give you great skiing weeks after a storm.  Of course, that wind can also turn pow on the upper mountain into wind buff, which while nice to ski, is still a disappointment.  Squaw also has more storm skiing options than Mammoth, where the top can be closed for a long time.  I would rather ski blue bird big days at Squaw, although Mammoth rocks on big pow days too.  Squaw has more sick terrain, although Mammoth also has some steeps.  I think either place will give you access to some terrain that will test your steep skiing ability, so have fun.
Both places are among the most consistent out there for snow fall.  
As I said before, Squaw, Mammoth, Snowbird.....Those places all give you a very good chance of having good conditions.  They all are fun even if there is no fresh snow.