Eeny, meeny, miny, mo.
I normally take into consideration how much free time I have at that point in life and will pick a pattern based on the time I have to actually sit and sew. If I don't have much time I will pick a quicker pattern. If I have a bunch of time I might pick a pattern that is going to take a little bit more work. I also think about what kind of fabrics I am in the mood to work with. Like right now I really wanna finish up the quilt I am working on so I can start on a 1930 repo. quilt!