You're going to want to look for an agency that can work with you on the overall technical strategy and not just the mobile app. Too many times, dev shops will not think about the big picture when building, which can lead to increased development and maintenance costs in the future.
You want to find a shop that can add some good information in the area of user experience. They may not be the main designer for your app, but you want them to be able to think about the user as they are building.
Ultimately, you want to find a partner to help bring your vision to reality, not just some company that will build to your specs. This company may not be the cheapest option, but in the long run, it will pay dividends.