I don't know anyone whose spring break is in early March. My timeshare week at Snowbird can start as early as March 5 and end as late as March 18. The later weeks of that range overlap with Ontario's spring break, which I doubt has much effect on Utah. In general spring breaks are spread over a 4 to 5 week timespan and never generate the crunch that Christmas does. Then there are places that get President's week off, so I think it's hard to make the argument that March is a more crowded ski month than February for the vast majority of ski areas. I would agree that post New Year's to maybe first week of February is less busy than later.
I've had a disproportionate number of warm sunny weeks during that timeshare. Snowbird is one of the best places to ski in that scenario because there is so much steep north facing terrain that retains a packed powder surface even when it's warm. There are parts of the mountain that can have difficult surfaces, but I've been fairly lucky to have very little of the "widespread refreeze with no warmup" scenario. Usually what cools down the weather in LCC after a dry spell is the next storm, which in LCC will often bury everything.
SnowbirdDevotee is correct about powder incidence being the same mid-December through mid-April. Mid-December can have the risk that there's not enough base under the new snow. For the past 17 years Snowbird averages being 73% open mid-December and 1/4 of seasons are 55% or less open. Last weekend had epic powder in LCC but only half of Snowbird's acreage was open. Alta was great, but if I'm flying in from afar I want BOTH Alta and Snowbird to be fully open. I'd rather live with working Snowbird's potential spring conditions in March with the whole mountain open. And of course the powder days will be better if all the terrain is available.
Park City and Snowbasin have less natural snowfall, lower elevation and more east exposure. lf you're booking far in advance I would confine trips with lodging base near those areas to the core period of mid-January to mid-March, avoiding the considerable risks of incomplete coverage earlier and sloppy surfaces later. Of course, one of SLC's key virtues is that if the "less reliable" places are good when you're there, it's still no problem to ski them in less than an hour's drive.