I finished A&P I-II and am taking micro this semester. I would have to say micro is harder because you have to study to understand, as opposed to A&P, which is mostly memorization. Time wise, I would have to say they require about the same amount of study hrs. It's hard to say which is harder because the difficulty level depends on the teacher a lot of times. I don't think the information is really difficult in either class(the volumn makes it difficult). In A&P you tend to concentrate on one system at a time, whereas in micro, you learn about a lot of different classification, diseases, and processes at the same, which makes it more confusing and difficult to organize and regurgitate for tests.
As far as chemistry requirements in micro, you don't really need a chemistry bacground because what you cover in micro is really basic chemistry(you do dabble in organic, bio, and inorganic chemistry but you don't need a chemistry background to understand the info). If your school requires that you take chem before micro then I guess you will need to; but I don't think it's necessary to take chem before micro to do well in the class. In either classes the key to doing well is consistent study habits, something I don't have, which can make memorizing 5 units of micro a few days before the exam kind of an adventure.
Don't underestimate the class, and don't psyche yourself out also, just "consistant" study habits and you should do fine. Good luck, B.T.H