Some people have AMAZING O/C'ing with there cpu's and chipsets.. its hard to duplicate sometimes unless you get the right stepping of CPU and motherboard combo (and motherboard Bios revision)... its do-able but not that easy and I hope that you have time on your hands...
But to the original question.. I would take the 2.4 over the 2.8.
dont exactly know the stepping but i built a 2.8c system a few days ago and i easily got to 3.53ghz on stock cooling. 1ghz front side bus and ddr500 ram did the trick on an asus p4c800 motherboard.
personally i prefer the 2.8c because it has a bit more headroom. i bet if i really wanted to, i could have pushed the cpu past the 3.6 mark.