It all depends. If you want long term success vs. immediate income, I highly recommend doing what you suggested and start something of quality first. Many have created half hearted sites and got burned out, because the promotion part was too much work.
If you have something that can carry itself once properly promoted, you have a sure winner and you can get to the point where building content and managing the site will be the main part of you work.
If you are an expert at marketing and have a lot of sites, you can definitely focus on that as well. It really depends on where your strength lies and then I do suggest to put your best foot forward. Whichever one that foot might be.