I've had a lot of success using the best available defense against the worst offense any given week. It's a little bit of a balancing act at times, and doesn't always work, but usually avoids really bad weeks. Sometimes, the best you can get is OAK vs SEA or something, but I still like that better than, say, CIN @ IND, for instance. I try to look a few games in advance and see which free agent defense has the best matchups for a few weeks and land them, sometimes carrying two defenses for a couple of weeks.
With that said, I think that your best choice of these two options is clearly AZ @ STL.
12 team, H2H, Standard Scoring (Starters in bold)
QB: P. Manning, Luck
WR: Julio Jones, Welker, Alexander, Hilton, Shorts
RB: Rice, Wilson, BJGE, LeShoure
DEF: DET, SD