ORIGINAL: Worc
They would be pretty close in flattness from factory loads. Here are the flattest factory loads for each from my ballistic calculator. I also threw in a .270 WSM load too.Notice the .300 has more KE also.
http://img.photobucket.com/albums/v701/Worc/WSM.jpg
Your not comparing the same weight bullet...so of course the .300 would have more energy...