Errm, I think he was asking which chip. Without knowing which stepping each chip is its impossible to say, but even then the difference will be minimal. TBH go for the cheaper of the two, as they will deliver a very similar result.
Actually the new boards with crap overclocking are completely different. NF7-S is the good one, NF7-S2G and NF7-S2 are the bad ones. But yeah, you're more likely to get a bit higher witha 2600+M.. I think on average they seem to top out around 100MHz higher.
there has been some very good 2600+'s lately, but that not to say there isn't any 2500+'s that can do better