It think it depends more on the quality of the content but I believe Q&A sites have a few built in advantages.
Most Q&A sites have far better architecture with a well thought out taxonomy. This can help create internal link 'neighborhoods' on specific topics and (if done correctly) can increase crawl efficiency.
Traditional forums can often get swamped with off-topic discussions that might water down the topical relevance of that page. Q&A is usually more narrow in focus and that traditionally results in very relevant answers.
Google has done some significant work on understanding both types of sites. Bill Slawski unearthed the patent behind this effort.