Who do I start at my flex position?

I am in an ESPN Standard League. I am starting Brees at QB, Slaton and Addai at RB, Sims-Walker and Vincent Jackson at WR, Olsen at TE, Prater at K, and PHI on DEF. I have a flex position remaining and I can't decide who to start. Do I start Pierre Thomas or Steve Smith (NYG)? Thanks