This will probably be dependent on two main things:
1) Institution. Each class is handled differently. Biostats was considered a joke class at my institution, but that's partially because mainly only bio majors took it so it never really got "that" quantitative compared to other schools where biostats may have mostly statisticians in it.
2) Aptitude. Are you a visual person? Good with quantitative topics like physics and calculus? Success in biostats pull on different skills than success in organic. Whereas organic is often mostly effort/memory based, biostats can be a lot more mathematically/technically challenging (performing calculations, using analytic thinking to solve some contrived case, etc)...I say can because statistics is often poorly taught and professors can turn it into a memorization grind if they like.
Personally, I found organic to be more challenging, but that's because the biostats often was simply a matter of recognizing which test to use in which case. It rarely really forced me to ponder and struggle with a set of data to figure out what needed to be done, though some classes may do this. Like I said, this is way too university/class/professor dependent.