Almost always shoot for RBs, but that depends on a lot of things like league depth (deeper leagues mean more sparse RB depth and more comparably usable WRs), scoring settings (if it's a points-per-reception league or one that rewards return yardage, then you can get value at flex out of some lesser receivers that might be better than your average third-down back like Sproles, etc.). The last thing that could affect this is the "OP" position that's gaining popularity in leagues, where you can start any offensive player in your flex spot (which favors a second QB at least 95% of the time).
Hope this helps, and feel free to chip in with your league settings, size, etc. if you want help applying my post to your exact situation
