Start with the idea you have the strongest gut feeling about. And definitely start with the idea that you will enjoy working on the most. But, don't let the enjoyment of working blind you from any reality about it. In your time off, try to fail it as fast as possible. Try to come up with solid reasons why it would fail...but truthful reasons. Not ones that have anything to do with the amount of effort or passion on your part. Contact potential customers. What do they think about the idea? Would they pay for your idea? Don't let them just say that they would, many people are afraid of hurting feelings and that's why they say 'yes'. Try to get them to pre-order your product. The way I see it, without this or a strong gut feeling based on reality, there really is no reason to start manufacturing and wasting money. Do with as much with as little as you have.