As a product designer, I get asked this question by clients often. Start by asking: Who is your user? What will this product help them accomplish? What does your brand offer in this space better that no one else can (your gift)? Are there inherent qualities in either platform that you must have in order for this to work? (For example, an accelerometer.)

It's less about pros and cons and more about proving your MVP through a technology that's most useful for your user. You may start focused on one and eventually build out the other, if you can prove the concept. Take the Spring shopping app as a great example.

The awesome team over at Crew built a tool to help. One of the main concerns is budget, but if you have a technical cofounder, you are one step ahead. http://appvswebsite.com

Let me know if I can help more over a call and good luck!