There really is no difference between the two, the 955 will overclock basically the same as the 965. If you plan to overclock then the 965 isn't worth the extra money, if you do not plan to overclock then the 965 is worth the extra.
As you can see there's a 20$ price difference.
They are just the same performance wise,965 is a tad faster due to its igher clock speed.But 955 is a better buy as it is cheap and you can overclok\ck it to 965 levels without a problem.
I would definitely get the 955 over the 140W version of the 965, but since the new 125 W c3 version is known to actually be stable at 4.0GHZ without maxing voltage, it might be worth it if you overclock.
The C2 955 and C2 965(140W version) are the same exact chip with unlocked multipliers and have the same overclocking head room, only difference is the 965 has a higher stock multiplier set. However the new 125W version of the 965 is on the new C3 stepping and has been proven to overclock a little better(most chips hit 4.0Ghz stable) and operates at lower voltages to reduce power consumption.
A big question is whether you would mind OCing. If you are ok with OCing, the 955 is probably a better choice. If you would prefer not to OC, then the higher stock clock of the 965 is probably worth it.