First of all s939 has dual channel memory, while s754 doesn't.
Secondly, the new rev E chips are now available and OC great, while you can't get these on the s754, unless you wait for the Newark (s754 San Diego), which will be released sometime in the future.
nF4 chipset also gives you the option of PCI-E.
And an option to go dual core in the future if you want.