Tough choice, but great choices.
Mac is in a more urban environment, near a big city if not exactly in one. Are you more of a country mouse or a city mouse? Carleton is near a very small, quaint town. If you prefer bigger cities, you might get bored at Carleton. But Carleton has a beautiful arboretum, so if you like nature and quiet, Carleton is the place for you.
If you wanted to work during the school year, I think Mac would be better since it is in the state capitol, closer to businesses and government, and even near several large Unis. But these are not necessarily biology jobs I am referring to.
Carleton is on a trimester schedule, so while you only take three classes at a time, those classes move quickly.
This is entirely subjective, but I think of Carleton as being more introverted, Mac as more extroverted. Neither is better or worse, just different. Which environment would you prefer?
I found them both to be very friendly and supportive. Students are welcoming and open minded in both places. Mac seems more politically engaged.