The price difference between the two versions of the chip is SEVEN DOLLARS. Seven dollars for a chip that uses 10w less at load is well worth it to me.
The 955 is 168.99. The 965 125w is 196.99. It comes down to what you intend to do with the cpu/machine. If you are not going to overclock then ask yourself is it worth for to pay almost 30 dollars more for 400mhz. I would. If you're going to overclock them would it be worth it to pay the extra 30 bucks for the 965. I would say no.
^Its actually $30 more for 200 MHz (3.4 GHz vs. 3.2 GHz), which does not make sense in a price/GHz ratio (normally we can't do this for cpus, but since they're both in the same family, we can) 965 has $/GHz ratio ~$58 where the 955 has a $/GHz ratio ~$53.