Some people will hate me for this, but I would not go to Steamboat. If you want a great mountain, there are other choices.
I lived in Squaw for 5 winters, with the first one coming during my 5th year of college. I blew my knee out during my 5th winter, so I left and went to grad school. You don't need to stay 5 winters, but I would try to do at least two.
Keep in mind that I have not lived in Squaw for 5 years,so some things might be different. The fact that they are now selling cheap passes has led to more crowded weekends than when I was there. The storm days are supposed to be more crowded too. When I lived there, I never waited in a 2 hour KT line. On normal days, you could do a million KT laps, and just basically walk on . I'm sure it is still that way during the week. On the normal(non pow) weekends I would not think the lines would be all that bad, as the mountain is big enough to spread people out.
I absolutely love Squaw. The terrain is insane, and they get a lot of snow. I do miss my time there, but I am not going to go back to being a ski bum. Keep in mind that anyhwere can have a bad season, but I would whole-heartedly recommend Squaw. I miss the awesome terrain.
I now live in socal, and have an office job. I ski Mammoth about 3 weekends a month, and hit little Mt. Baldy when I don't go to Mammoth. I love Mammoth. It gets a ton of snow, and has lots of good, steep terrain. I like Squaw better though. I ski Mammoth because it is a 5.5 hour drive from Orange County, making it doable on weekends, while Squaw is too far. If they were the same distance, I would choose Squaw. Mammoth is great though, some of the best terrain in the country, wind which smoothes the snow, making for great conditions even long after a storm, and an elevation which ensures snow on the days Squaw gets rain. The town of Mammoth Lakes is right there, so I'm not sure what you are getting from Yahoo maps. Mammoth also has a great terrain park.
I would choose Squaw to ski bum, but you can't go wrong with Squaw or Mammoth. Either way, if the snow is good, or even just average, you should have a great year. Both places get around 400 inches in a normal season, so you should have a good time. There is certainly a chance you will have a bad year, but the snow is usually pretty consistent.
Good luck with whatever you choose. Like I said, both Squaw and Mammoth give you good chances of living the ski bum dream.