We did a Stanford campus tour last summer so we know a bit about what Stanford is looking for.
AP (or IB) classes are almost a must to be competitive in applying Stanford or, for that matter, Ivies. Most top students have 3-4 AP classes under their belt. Some, a lot more than that. Of course, you have to take AP tests and do well on them too.
As for the question of which is better: getting B on an AP class or getting A on a normal class. The standard answer is that colleges prefer that you take AP class and get A. Some school gives extra point for AP/honor classes so it is a good way to boost the ranking of the student. On the hand, you don't want to get C or D on your AP classes which will back-fire.
No, AP is not horribly hard. My older daughter is take AP chem as a 10th grader. It is a lot of works but nothing unmanageable. We will see how it goes with the AP test.
