Who do I start and who do I sit?

Two questions. 1st, do I play Deshaun Foster against a stout Tenn defense or Play Selvin Young against a porous Det defense, even though T-Hen looks to start. 2nd, who is the better flex play: Adimchinobi Echemandu against Oakland, Marques Colston against Jacksonville, or Justin Fargas against Houston? This is a PPR league.