Me and my friend have started a metal band a while ago but we're still thinking about what metal subgenre we will be. I want to be thrash metal and he wants to be heavy metal. What direction should I choose?

That's for you to decide.

Why don't you stop worrying about what sub genre you're going to be, and just write good music. Then let elitist metal heads like us decide what it is, while you insist that "Its just METAL man!"

Especially since the difference between thrash and heavy metal is pretty small.