I get too many ideas (not all worth looking into, but ideas just the same) at one time not to work on multiple WIPs. I have one main project, but sometimes, depending on what I'm reading or doing with my life at the time, I feel more inclined to work on something else. I would say work on something until one thing in particular really grabs you, and then finish that one before going back to the others....?
I juggle several WIPs too. Sometimes when I've done all I can with one, it's that space away from it that allows me to see it with fresh eyes again.
Also, go where the passion is! I spent a very long time working on a novel that I wasn't ready for yet - hard to describe why, too big a concept, and I felt like I needed to grow a bit as a writer first - and it was coming along painfully slow. Then when I gave myself permission to write something else, the new book just flew out of me! I'm very glad I allowed myself to take a break from that first project, or I may have still been poking at it and going nowhere.