Well, as a huge admirer of one and someone disgusted by the other, we can start with the obvious...high school kids. Yes, BA also has the occasional report on high school prospects, but nowhere near the over-the-top hysteria shown by Rivals and its ilk vis-a-vis recruiting. Also, BA's focus is on draftable high school prospects...rarely, if ever, do they discuss recruiting races for college-bound ballplayers, saving that sort of thing when ranking each school's recruiting class (Go Vandy!)
Also, a goodly portion of BA's writers and contributors have hard sports reporting backgrounds. Rivals tends to be a bit of a grab bag, making it hard to discern one's intentions. Anyone who thinks that at least a few of these recruiting "gurus" is working as an unofficial coach for the school of their liking must still believe in leprechauns.
BA's analysis focuses mainly on a professional sport, and while high school kids do get drafted, once they sign on the dotted line they become professionals. If someone wants to critique the play of a young minor league prospect like, say, St. Louis' Colby Rasmus, well, that's pro sports for you. But this making a big deal out of where a kid signs to play college ball shows some of the worst of our sporting nature...that anyone would go to a message board or call in to a radio show to criticize where a 17-year old kid decides to go to school is sick.
And of course, we're comparing two different animals. The appetite and passion for high school and college football in this country dwarfs the interest in college and minor league baseball by a truckload. The bigger something becomes, the more reptiles are attracted, and thus comes a greater risk of something serving the reptiles, such as Rivals.