I'm trying to remember why you need a function key in BF3, or even the last time I used one in game, or just the last time I used one at all. Voting?
You should really just look at the layouts of both side by side, and take note of all the function layer keys; which ones you think you'll need/use and how comfortable they may be for you to use. I have the found the Pure is great for me but a lot of others don't seem to agree. And also keep in mind the Pure has a near standard layout of keys, bar right shift and tilde, while the Race has a non standard layout.