“Better” and “worse” are subjective measurements with no scientific value whatsoever, and therefore are right up Dr. Boli’s alley. You have come to the right place.
Polonium has the edge in practicality, owing to its traditional use in the heads of polo mallets, whence, of course, the name of the element.
Americium is more widely found in ordinary households, however, since it goes into smoke detectors. That means that any one of us might have come into contact with it and be carrying around an atom or two of americium even as we speak. Et in Arkadiy ego, to coin a phrase.
Which one is better or worse therefore depends on how you feel about polo, or, contrariwise, smoke detectors.