I'm riding a flag and it definitely is stiff! I'm loving it, but if I were going to buy an all mountain board, it would definitely be the slash (I may buy it next year to start hitting jumps better, and to better my switch riding skills). I've heard good things about the agent, and I think Rome stuff is top notch quality.
The slash will definitely have more flex. Check out Rome's website, they have a great breakdown of their board tech and it should help you decide which one suits you.