If we are in the business of making assumptions, there is no dichotomy, you can as well consider both hypotheticals. (Actually believing that either of these holds in general, or in any given case where you don't have sufficient information, would probably be dumb, ignorant, a mistake.)
This misses the point a bit due to an equivocation on "assume". In ordinary discourse, it usually means "assume for the purpose of action until you encounter contrary evidence". That's very different from the scientist's hypothetical assumptions that are made in order to figure out what follows from a hypothesis.
Another monthly installment of the rationality quotes thread. The usual rules apply: