Well, theoretically, the 2600+ should OC higher because of the higher default clock speed. However, both should be able to hit at least 2.3-2.4ghz with ease. Depending on your system and cooling, you might be able to get 2.5ghz out of the 2600+.
If money is a issue and you're happy with 2.4ghz, then I'd say get the low-wattage 2400+.