I don't think there is any way to measure the hysteria around them. However, I do think Rob has a bigger fanbase. Whatever reason, I'm not sure. It could be his looks (which should not be the case), his personality, his movies.
Especially right now, I think since Water for Elephants is out, it's adding to his "popularity."
Though, Taylor's new movie Abduction might bring Mehr attention. Even though I don't think it looks too promising, I'm sure people will see it. :)