Hands down AFF!
wattpad seems more professional(since majority of their books are geared towards public publishing and are not restricted to just fanfiction) but somehow I find my time on aff more because I feel it's hard to socialize in wattpad compared to aff.
AFF seems more than just a community. We're all one big family and wattpad just seems like a business tbh. (even though everything is free and all)