I have a x2 4000+ and I am able to clock it to 2415 mhz, no problem at all. I do have a thermalright venus HSF, so I don't know how it would do with stock HSF. But, I never go above 30c. I got to 30 when I was encoding a DVD to Divx and playing a game at the same time. Otherwise, I am at about 15-20 idle, 20-25 normal loads. And with the OC, since the multi on the CPU is 10.5, I get my ram up to 800 mhz, instead of 700 at default speed of 2100 mhz core. I could go higher, I am sure, but my mobo has a max fsb of 230.
I would go for the slower one, and OC. But, if you don't want to OC, you really won't notice a difference between the two CPUs.