As most questions without context, the answer is, "it depends".

What type of app would you be developing? What would be your orientation for the app? (gaming, productivity, business, educational)

That type of information can give us a ground to tell you what mobile OS would best fit for your goal. But for most startups and mobile oriented business, you would pretty much need an app for both OS.

Both OS are secure, iOS however focus on security and updates, Apple's old phones still get updates and security patches, that makes it most secure, also it's sources aren't available to public that adds up to security of iOS. I have an app for cleaning service. I worked with IDAP company for it creation, and we decided to use iOS(link), quite good decision.

Everyone knows that iOS is better than Android. For security purpose also iPhone are more secure than anyother Android Phones. Studies have found that a far higher percentage of mobile malware targets Android than iOS, the software than runs Apple’s devices.

Thanks
Thecrazybuyers - One-stop solution for iOS and Android Accessories.

I think the best OS is IOS or Blackberry OS