Mt. St. Louis is great for bombing first thing in the morning when freshly groomed (if you see a 6ft tall+ brown guy on a prior race board, that's me)
And as noted, it's great for the whole family. but can become a bit repetitious run wise. The park from what i've seen is always in top shape (i dare not go near it though).
Blue, great on the weekdays, horrible on weekends ---> standing in line 25mins for a 45second run... NOT FUN.
like Oldman says, go during the week, you have the entire place to yourself! And when it snows, Blue does get a good dump (they're far and few between, but can be good). Also, and as mentioned, the apres is much better. Also, the North side of Blue has some interesting and steeper runs with little pockets & areas where you can duck in and out of some glades (yes on the north side), off Hogs back duck right into the trees, it will spit you out into Happy Valley. You can do the same coming off Willys run, about 1/4 way down, duck left into the trees. Oh, and if you're adventurous skip buying a lift ticket and hike the unopened area south of the south side. With a good snowfall those runs are choice!
For me, if it's for exercise and training i go to Mt. St. Louis. If it's a weekday with the buddies blazing and drinking, Blue all the way.