At many schools Bio is a VERY popular major so you're competing with hundreds of other bio majors to get research opportunities. Thus, it can be more difficult to find them than in smaller departments.
I think that being a chem major did help me because the major makes you take classes that are generally considered very difficult (p-chem, extra organic chem, etc.). This gives your gpa some kind of backing that bio majors don't get. Engineering/physics majors also get this advantage.
Either way do what you think you will enjoy the most and do the best in. What made me choose chem was that bio involved so much straight memorization and very little actual thinking (though this seems to be what the first 2 years of med school are about anyway), while chem was all about thinking through problems. Also, chem gives basic explanations for stuff, while bio sometimes gives results with little explanation.