Who to start at RB and Defense?

I have a problem. I have Portis as one of my backs and he is facing the Cowboys D this week which is the Def. that I start. I also have the Broncos D and J. Stewart (who has been a TD magnet so far). Seeing that if I start Portis & the Boys, everytime that Portis scores, the Cowboys will lose points. Should I start the Broncos D since they are playing a horrible off. this week in KC? Should I start Stewart since he is facing a horrible Atl. def this week? Or should I just stick with what I got starting with Portis & Dallas D?