Well as a BJJ'er I certainly have a lot I could add to this but the thread is fairly long here...I just have one thing in response to Chris, and Ledyard Sensei,
Hopefully, (and I know this isn't always the case) a well trained BJJ guy would never attack anyone just like some one in Aikido wouldn't. BJJ is ment to be a defensive art.
What you have to think about is that 97% of Adult males in the US either participated in Football or Wrestling in highschool and possibly on in college, that means one thing--Tackling! Its not really about BJJ vs. Aikido...its about aikido vs. large, angry men diving at your legs!
If you are really wanting to know for sure...find a friend that used to wrestle or play football, go to the mat and see if you can avoid getting taken down..simple.
For any other points I might make..I will simply add this link to a great interview regarding Aikido and BJJ.