I generally let life decide this for me. Interest, traction, whatever can cause one project to suck up more of my time than the other. At some point, this becomes a formal decision to stop sinking resources into some half baked idea that is failing to launch and get more serious and organized about the thing that has legs.
a mix of the following works for me:<p>1. resources (time) required to get mvp together to test assumptions<p>2. amount of long-term work needed to get foothold in market<p>3. total size of market vs costs over growth -> projections<p>4. if you fail, what have you lost? sunk costs vs variable ones.. redeemable experience at the end..?<p>5. most importantly, which actually interests you more?