I just wrote a long answer about accepting answers, and realized you asked about voting. I would vote for both; why choose? Unless there was a long separation, which would indicate copying or at least not-reading, the answerers did equally good jobs and deserve the votes equally.
To salvage some of my original answer: I almost always accept the first to post. The primary goal here is to come up with good answers. But given the choice between "good answers quickly" and "good answers slowly," quickly wins every time. Fastest Gun in the West is a problem if and only if it leads to poor-quality answers. Of course, if the posts aren't effectively identical, or aren't helpful/correct/useful, this paragraph does not apply.
It's not an ironclad rule. If the answers aren't comprehensive, I might wait a few minutes before voting/accepting to see how the edits go. And if one of the answerers is someone I recognize as a solid performer but has low rep due to being new, I would give him the edge in the awarding of the coveted check mark.