Hmm, I really think TF should make some sort of hardware FAQ and update it monthly or something like that, I'm sure many members with the right know how would be willing to donate their time...
Anyways, the big difference between the two sockets is compatability. Socket 754 is basically discontinued as of now, so the CPUs you see on the market now are the only ones you will ever see. Basically, if you want to be able to upgrade in the future, then socket 939 is the way to go.
Socket 939 also uses hypertransport technology speeds at 2000mhz, whereas socket 754 hypertransport is only 1600mhz. The only other big difference is socket 939 is dual channel compatible, and socket 743 is not.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w