Aittokallio has the highest ceiling and is probably the most talented of the bunch.
Pickard slaved away for Seattle (the worst team in the dub) all his WHL career and got a lot stronger. I think he's the most sound goalie the Avs have right now.
Patterson and Millan are both kinda hit-or-miss when it comes to goalies. Both show strong strong strong careers in the NCAA but both also show some mental weaknesses.
But all-in-all the Avalanche have a wonderful goaltending network.
In my mind this is the career potential:
Pickard (Starting Goaltender)
Patterson (Backup goalie; career AHL'er)
Aittokallio (No. 1a goalie in a goaltending tandem)
Millan (Career AHL'er)
But all show the talent necessary to play in the NHL. The biggest part of being a goalie is mentality and the NCAA guys haven't really proven/developed enough to be considered strong in that area... Even though Patterson was bombed more than London in WWII in the NCAA tournament .