Like they said below I would check matchups, but overall I would say Jags D. They are about even, but the Bills Offense is not as good as the Jags so the bills D will be on the field longer. Also they're just seems to be a sense of instability in Buffalo which leads to inconsistent play.

I assume you mean your backup defense. Remember, you're only playing them for one week. I would look at the bye week for your starting defense and see who has the best matchup. But I never carry a backup defense on my roster, I'd rather carry an extra RB or WR until the bye week, then I drop my worst guy and pick up whoever has the best matchup. After the bye, I drop the defense. I do the same thing for TEs and Ks. No need to clutter up my bench with those positions.