What are your starting positions in the league?
If they are Standard 1QB 3 WR 2 RB 1 Flex 1 TE 1 K 1 D I.E. you can start 1 more RB in a flex spot I would go after Portis. You have the second most money to spend, may as well spend some money on a big name. If the Portis bidding gets a little to rich I think Kevin Smith will be another good option for you, he's looked fantastic throughout the preseason and has little competition for carries in Detroit. Not to mention he's shown a great aptitude for catching passes out of the backfield (which OC Linehan seems to love)
If however you don't have a flex and cannot start another RB I would go with Boldin and really solidify you receiving corps.