Well, both are free sources of obtaining links.
If you are doing article submission then you should concentrate on writing quality articles then submitting to top article directories this way you would get your article indexed in search engine faster.
If your article has quality then it would be published by other webmasters also meaning one article submission is giving you multiple back links to your site.
Article submission is also very helpful in getting targeted visitors to your site if you have used your site link in the article.
On the other hand, forum link building is also very beneficial, I don't know if it is true but I have heard that forum content is indexed fastest of all, if it is so then it is very helpful in getting back links to your site.
The main benefit of forum interaction is it would give your company, your site brand name popularity if you have contributed to the forums.