I went through the same decision making process about six weeks ago re 2.4 vs 2.6. The consensus, which I agreed with, was to go with the 2.4c. I now have a SL6WF (not an MO) 2.4c and it does 3.2 Ghz (267 Mhz FSB, 5:4, Mushkin PC3500 BLII) with complete and total stability on air and default voltage. At 270 MHz FSB I start seeing a few errors when running Prime95, even with a slightly elevated core voltage. A 270 MHz FSB with a 5:4 ratio is about the limit of my memory without incurring Memtest86 errors. It seems I have a good match between CPU capability and memory capability. BTW, I did bump the memory voltage up to 2.7 volts based on memtest86 results at 270 MHz.
Here are the things I think you should consider.
First, the max capability of the 2.4 and 2.6 are probably the same.
Second, the only real difference between the two is the multiplier. The 2.6 has a higher internal multiplier (13 vs 12) so you will achieve the max core speed at a lower FSB speed. This means you will not be running/overclocking your memory as fast as you might with the higher FSB needed for the 2.4c (for the same core speed). Therefore, you are more likely to have your CPU be the limiting factor with a 2.6 and not be able to utilize all the potential bandwidth of your memory.
Of course all this depends on the luck of the draw and the capabilities of the specific CPU/memory you get. Obviously some CPUs do better than others but it is my opinion this currently has little to do with the labled speed rating. However, based on my experience and that of others that I have seen posted here, the 2.4c would probably be the better choice.
Newegg appears to currently have stock on both CPUs and they are both about $170.00.