I selected "Other" because I think they both matter somewhat. I am of the belief that messageboard posts are informal writing, akin to the informal speech we use in most of our daily lives. I don't expect such on-the-fly missives to have perfect spelling or grammar, just as I don't expect people to speak with perfect grammar or pronuciation. I would never write a business document using such informality, of course, but on messageboards... no big deal.
However, just as people judge you by the way you speak, they will most certainly judge you by the way you write. If you're trying to be convincing or persuasive in any way, both spelling and grammar are important, even in an informal setting.
So... Other.