Biology is worthless as a major; as long as you take the pre-med requirements, they don't care what your major is.
Granted, Biology requires a lot of the same classes, so it might be easier to be a bio major if you're already pre-med, but the biology degree itself isn't worth anything.
Taking more irrelevant classes is probably harder (as I said), but because it's harder it is also more impressive and thus looks better on your resume (as rms said).