This is interesting. Thanks for asking this question.
I guess (maybe because of my job), I am an organized person by nature and very goal-oriented. I make a schedule and stick with it. Of course, quilting is a hobby for me, so I set a comfortable goal, and sometimes I even finish ahead of the schedule:)
I like to work on one "big" (relative term here as I am a beginner and haven't done anything bigger than lap size) or complex project, and may start a smaller, easier project just to keep me motivated. But I definitely don't start another "big" quilt, until the one I m am working on is bound.
I do, however, plan quilts in the middle of projects again to keep myself motivated. I design or pick a pattern, choose or buy fabrics and get it all together so it is waiting for me when I am ready.
As far as picking which one is next, I just go by my mood or the deadlines I set for myself.