harder to say when comparing different clockspeeds, but at similar clockspeeds, the s939 in only a few % faster than an s754. is that worth $100 to you?
basically the differences are:
Dual Channel Memory (s939) vs Single Channel Memory (s754)
Four DIMM's avail (s939) vs 3 DIMM'S (s754)
1000Mhz Hyper Transport Bus Speed (s939) vs 800Mhz (s754)
consumes slightly less power than s754's
939 claimed to run cooler than s754 ( tho my s939 runs the same temp as my s754)
performance differences have not been noticeable to me, tho benchmarks may show a slight advantage to s939. there may be an advantage in future upgrade paths, but not postive on that.