I think what we really need is some sort of automated bot for the message board. As soon as it detects any sort of "Aikido vs BJJ" post it deletes it then forwards the poster the dozens of threads that already exist on the topic.
I think overall Aikido is a better package than BJJ in the sense of atemi-waza, flowing movement, multiple attacker scenarios, weapons, and spiritual awareness. After 35....who needs the injuries!! I would be bored and a little uncomfortable rolling around on a mat with a sweaty neanderthal for 2 hours.
One, as Michael said, that's rather insulting. Two, your attitude and comments come across as completely ignorant of what BJJ is all about. I'd also agree with Michael in that I have found there to be far fewer injuries in BJJ than Aikido.
In regards to the question at hand, who cares? Why is someone trained in BJJ going to "attack" you? How are the going to attack you? It's not like there is one singular attack that BJJers perform. I'm assuming your actually asking: "what should I do when a guy shoots in for my legs?" The answer is: go to a BJJ/MMA/Sambo/Wrestling gym and they'll show you what to do (short answer: sprawl, whizzer, knees to the face). You'll get a basic understanding of what to do, be satisfied with that. However be aware that, more than likely, if someone wants to take you down, they will. If the the person going for the shoot grew up wrestling here in the States, forget it, game over, they are taking you down.