Both are pretty much the same with premium bullets, but the 7mm WSM offers slightly heavier bullets. As mentioned previously, the 270 WSM brass is easier to obtain, but Midway and other mail order firms offer the 7mm. To be safe regardless of which you choose, I'd order enough brass (500+ cases) to last a long time.
Basically, either will do the same job, but as a general rule, I prefer a 7mm (it is a widely-known scientific fact that the extra .008" of the .284" bullet vs. a .277" bullet makes the 7mm a REAL game slayer!).
