If you go to Tahoe you have several resorts. If you go to Mammoth you have Mammoth and June.
Normally I would say Squaw. I skied Squaw everyday for half a decade. I skied Mammoth 2-3 weekends a month with some longer trips in college. I am now an office dweller who skis Mammoth 2-3 weekends a month again, also with some longer trips.
If I had to choose between the two I would take Squaw. Mammoth has great terrain but it simply does not have the steeps of Squaw, either in terms of challenge or variety. Don't get me wrong, Mammoth has plenty of stuff I can't ski, but Squaw has much more, and better, terrain in terms of challenge. Squaw is also a more featured mountain, with more cornices and small to medium drops, as well as the huge cliffs.
Squaw also probably gets more snow, although that is debatable. Squaw measures at 8,200 feet, which is 2/3 of the way up the mountain. Mammoth measures at 9,000 feet which is 1/3 of the way up that mountain. Squaw gets more snow at 8,200 feet than Mammoth does at 9,000 feet. Overall they both get lots of snow. Where Mammoth has an advantage is in the fact that it is higher, sometimes Mammoth will get snow when Squaw gets rain. Overall I would call snow a wash, with maybe a slight advantage for Mammoth. It really is close though.
Both mountains get lots of wind, but Mammoth gets more. This is a blessing and a curse, you can get great windbuff turns weeks after a storm, which is good, but the wind can also turn a pow day on top into a windbuff day, which is bad. This happens at Squaw too, but less often. You probably get more pure bluebird pow days with good pow at Squaw than you do at Mammoth. I would also rather ski Squaw on a pow day because of the terrain quality, although I have heard the crowds are worse now than when I was there which dampens things a bit. Mammoth gets less bumped out than Squaw because of the wind, so sometimes better turns will be had at Mammoth.
Mammoth has a better park, although that does not matter to me. I believe Mammoth also has better backcountry, although I am really not qualified to judge.
Squaw has 360 degrees of sun exposure, which means something is usually wintery and something is usually slushy. This is a good thing. Mammoth has less sun exposure off the top. While this can be good, if things get slushy and freeze you can be pretty fucked until the next storm. This happens at Squaw too, but Squaw usually has at least something that will get slushy when this happens.
On the pow days when the top is closed I prefer Squaw. It has Red Dog and KT while Mammoth has only chairs 22 and 25. Don't get me wrong, those chairs have great terrain, it's just that Squaw is a better bet for days when it is storming. Once again, the crowds have supposedly gotten worse at Squaw so this might be a moot point.
In a bad year I would take Mammoth. In an average year or better I would take Squaw. Squaw is more of a crapshoot, but if it fires it is better than Mammoth.
One more thing to consider is that Alpine Meadows is right next to Squaw. It is one of the most underrated mountains anywhere, in terms of terrain in blows the doors off places like Vail and Park City. Alpine has great hike-to stuff, much like Squaw. Sugar Bowl is also accessible
Kirkwood is another sick place. It has sick terrain, as good or maybe even better than Mammoth although it is not quite as big, and it gets more snow than anywhere in California. It is out of the way, so it is tough to hit on a trip where you are staying in North Lake Tahoe, which is where Squaw and Alpine are located.
Hope this helps. Overall I really miss skiing Squaw. I did not really miss Mammoth when I lived in Tahoe, so take that for what it is worth. I have never been a Mammoth local, although I am there a lot and have put in several hundred days there.
If you can I would go in February. The base will be better. It could work for you either way though, you really just have to guess and hope.
For this year I would choose Mammoth. Mammoth got 60 inches at the top a couple of weeks ago and 30 inches maybe a week ago. The base is all set. The 60 inch storm was mostly rain at Squaw, so I think Mammoth is more of a safe bet for this year. That could certainly change though, things are still early.
I should add that Mammoth has great public transportation, if that matters.
Good luck. Obviously I love both places. You can have a great time wherever you go. Like I said before, Mammoth is more of a safe bet but if you get equal conditions I would choose Squaw. Tahoe has a variety of resorts if that matters to you. I will say that I would avoid Northstar and Heavenly if terrain is of high importance. Heavenly is okay, but it does not have the terrain of Mammoth/Squaw/Alpine/Kirkwood or even Sugar Bowl. If you stay in South Lake and have a car you can access Kirkwood and party in the casinos, although I would not waste my time partying on a ski trip.
Have fun. California has some incredible terrain and it gets lots of snow.
