Okay first, how much are both at your prices?
Okay here are the key differences on the sockets.
754 is older, and this only has 64 bit wide memory path's, only supporting single channel memory. This equates to half the memory speed.
939 is newer, and has a 128 bit wide memory path, theirfor supporting dual channel memory which is a 50% memory bandwidth boost.'
So 939 is a higher peformance socket, + more upgradeable. With SKT 939 you can get dual core CPU's as well as PCI -E motherboard's
Here are the core differences.
754 is NewCastle / Clawhammer. These are based on the old 130nm process, and these CPU's require ALOT more energy as well.
939 is for the main part Venice (Their are older cores, but these are rare). These are the new 90nm process, and can require half the energy of a clawhammer.
Now, for overclocking. 754 was at the end of it's life, with the speeds as high as they can go. 939 cores (Venice, Whinchester) may only come at a stock of 1.8 ghz, but since AMD are soo good at making CPU's, most of the stuff they release as a 1.8ghz CPU can be ran at 2.5 or more ghz. To make it run faster, we do something called overclocking