you can take both, although you aren’t allowed to take ab and bc together int he same year (see the college board’s rules on that.) so you’d need to spend two years taking the tests, which is a disadvantage. the other disadvantage is that bc is merely an extension of ab. so if you take both tests, you’ve taken everything in ab twice, without getting any extra course credit for it. you only get additional course credit for the parts of calc bc that are not also in the ab test.