Yeah, I definitely do that one.
For me I have a history of leaving heaps of half-finished, fragmentary would-be novels in my wake, so I actually need the discipline of focusing on just one at a time. Not that I don't cheat a little, but I have to watch out for the urge to work on anything else rather than actually put the time in to get one project finished.
I think the main deciding factor for if working on multiple stories is the way to go is whether you can give them both their due. If you (and when I say you, I'm thinking of me) are just going to switch to something new the minute you start really getting somewhere on the second project, then it's time to reel yourself back.
For picking which idea to start with, let me recommend two approaches:
a) flip that coin, and if the result makes you unhappy, go with the other one.
b) Sit down to a blank screen/page, and start writing whatever's in your head right then.