Tosun is a better natural finisher, no doubt, but to play in the premier league in a modern formation (usually one striker up top), you need a lot more to your game than finishing alone. Tosun is slow, poor at holding up the ball and doesn't make runs in behind or give defenders much to worry about.
DCL doesn't have Tosun's finishing ability, but he creates a lot more problems just by virtue of his pace and strength. His hold up play has also gotten a lot better in recent months and he's created a lot more chances than Tosun typically does.
That's why it is incomprehensible to start Tosun ahead of DCL - Tosun is simply not as good an all round centre forward for a premier league game. Chuck them both in a Turkish league game and Tosun would likely find the net more than DCL, but at this level DCL is a far better shout for his other attributes.