Only got a day at Alta and a day and a half at Snowbird but I liked Snowbird so much better. At Alta I felt like you had to hike to get to anything decently good and most of it wasn't even open. They just didn't have that much steep terrain and it was all very long traverses to get to. Snowbird on the other hand seemed a lot larger and was a lot steeper and just a lot more expert terrain to find. Snowbird park is worthless, two boxes and two 5 ft jumps, don't know why its even mentioned.
Oh the best run at Alta was the 'Keyhole' which is a run from the top of the Wildcat chair in which you go over that ridge to get back into snowbird. Incidentily, it happens to be a run in Snowbird. Supreme lift was my other favorite part but I didn't want to waste my day hiking into Catherines.