Socket 939 will be replaced by Socket M2 (or in the case of mobile processors, Socket S1), which is the next AMD platform supporting DDR2-667 as of Q1 2006 most likely.
I think he is refering to the Pentium M when he suggests the processors are almost twice as fast. The Pentium M technically has about half the clock speed of the regular Pentium 4s and that may give the misconception it is twice as fast since they are both on par, but it's simply because the P-M has more IPS than the Pentium 4.
The P-M and Turion will almost certainly have longer battery life, less heat output and less power consumption than their desktop counterparts.
I am almost postive the P-M supports SSE3 instruction sets, but am not sure if the Turions do or not. Either way, it shouldn't be a big deciding factor in your purchase
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