I'm curious how people determine how much defense offsets offense.

Obviously the rest of your line up is likely a factors, but out of the following, which would generally be more valuable at 1B? Both have plenty of PA, and the rest of the offense is relatively good.

Hitter A: .906 OPS, 3 minus plays and .983 fielding %
Hitter B: .799 OPS, 21 plus plays and .995 fielding %

You could say that preventing 24 hits is roughly equivalent to getting 24 hits over 700 PA is about 0.34 points of AVG, so as a rough gauge, that's about 68 points of OPS if they are all singles.

That closes most of the gap. Then consider how many less innings you have to buy (is it just 8 innings?) and some undefineable number of double plays that come with better range.

Me personally I like the range guys, but this one is pretty close.

I use a similar approach to jfranco. Depending on what type of hits the OPS comes from (if HRs, yuck) and how they both normalize, I'd probably take B and throw the team in a big hitter's park to maximize the benefit I get from better range.