Based on your initial question about .308 and .300WM, and which has more power and is flatter shooting, I suspect that you don't have a lot of experience with rifles. Do you have any experience at all? What rifles do you currently own?
If you don't have any, or don't have much, experience, I'd really have a hard time recommending a Magnum for your purposes and I'd question your suggestion that you're planning on 300 yard + shots. Generally speaking magnums aren't a good choice for someone who doesn't have some shooting experience under his belt. Likewise for long shots. Those are for someone who has had a lot of experience/practice. And, practice is going to be a lot easier and a lot more fun with a .308 than with a .300 Win Mag. And, out to about 350 or 400 yards, a .308 is going to kill deer just as efficiently as a .300 WM.
I can't believe it took thre pages before someone asked this.
No to be picking but someone that is asking which one is flatter shooting between the .308 and 300 and which one has the most "knock down power" (no such thing BTW)obviously doesn't have much experience shooting rifles and really doesn't need to be taking 300+ yard shots on anything but paper especially with a magnum cartridge.
I suggest something along the lines of a 7mm-08, .270 Winchester, .308 Winchester, 25-06, etc... prefering the .270 then the 7mm-08.