I am looking at a Kestrel vs the App, which one do you recommend most?

We recommend the Kestrel over the use of a Phone. The Kestrel has a better battery life (15+ days with Bluetooth off), is more durable (it even floats), and is not reliable on cell service to gather weather information. So the Kestrel will work off the grid (so will the apps, and some like AB Tac are designed for off the grid use) and give you on demand, real time weather data.