FJax & Spiller owner here and I have the same dilemma, but I plan on starting Spiller over FJax this week.
It's a crap shoot at this point vs a tough 49ers D, but if both are at full health, I'm gambling that Spiller might have a better chance of breaking off a long run or catch in limited touches.
RB- J Charles, R Bush, G Bernard, S Jax, B Tate, Snelling
WR- L Fitz, Amendola, Gordon, Gordon, Blackmon, Broyles, Marlon Brown
TE- J Thomas, Cook