To be honest, the two should go hand in hand. There are many a time that freelancers will not need just a marketing skill but they will need to have improved skills that will help them to excel. There is no way freelancers that have improved skills will not need to have marketing skill. Personally, I will advise someone that is into freelancing to have exceptional skill first before thinking of having marketing skills. This will save the time more than the other way round.