If you were to try and determine the relative strength of either of the links, I would assume that both would be about equal... However the blogroll link will stay there indefinitely, while the post will get archived eventually...
If you had the option, why not get both? That is, you can get a listing in the blogroll and then have them write a post about why your there... That would add relevancy...
Hope that helps, just my 2 cents.