They're both good boards. K2 has been making boards longer than most boarding companies have even existed and Forum is owned by Burton.
Forum had some crazy quality issues a few years ago but that seems to be a thing of the past and K2 suffers a bit from its hardcore ski stereotype but none of that should be an issue affecting what board you should get.
K2 makes boards for just about every style of riding, while Forum seems to focus more on the park end of things. Do your research and pick the board that best suits your style of riding and you can't go wrong with either company.
Out of those two companies I'd have to go with K2 personnally.