Easiesness is entirely dependent on school and particular professor. Ask students, see the syllabus(some profs will give easy points like participation or HW assignments to boost grades) and other info like that.
Both classes are very helpful. It's true alot of cell bio can be covered in a good biochem class which you will essentially have to tkae, but I don't necessairly think you need to take a genetics class to learn the info tested on the MCAT. My personal experience was genetics class went pretty in-depth with the material and the tests were full of problem solving questions which while a good learning experience don't necessairly resemble what you'll be tested on on the MCAT or later on in the field of medicine(I remember for my class there was a good bit of math needed for these tests) and aren't always the kindest things to GPAs. Just food for thought; unless there is a clear discrepancy in your school of one being much harder or taught much worse you can't go wrong with either.