Bikemig, your framing the topic in either/or choices is unnecessary. You work with what you got. In very wet rainy conditions waterproof bags provide a first line of protection with subsequent layers afforded with plastic or waterproof sacks. In drier hot conditions it's less of an issue as the bag can dry out. You wouldn't put wet things with dry things in either bag nor would you keep wet things bundled up all day in a hot dry bag.
I'd much rather have a bag that sheds water in very wet conditions than one that collects water.