askubuntu (and every other stack exchange site) is designed for questions to which there is a definitive answer. Questions that either dont have a single answer, or the answer is likely to be subjective or opinionated, arent really for ask ubuntu.
The ubuntu forums, whilst you can still ask specific questions (and get good some very good answers), its also possible to start debates and / or more argumentative questions.
As to which is best, that depends, to get something done or to get something to work, ask ubuntu is probably the better tool. For reasoned debate, head for the ubuntu forums.
for more info on what you can ask, see here: https://askubuntu.com/faq