Cost is difference is specifically when and where you buy it. I just picked up a G752VY DH72 for less than the G751 DB72 could be bought anywhere. $200 less.
The specs on a lot of sites have the incorrect drive size. The G752-DH72 comes with a 256GB Samsung SM951 PCIe 3.0 NVMe drive, G751-DB72 has a 256GB Samsung XP941 AHCI for PCIe 2.0 that is only capable of about half the speed of the SM 951 NVMe.
G752 has 2xM2 PCIe Gen 3 slots with NVMe support. G751 has one and does not support NVMe. Some have said its possible with a BIOS hack but no such results have been posted yet.
G752 has single SATAIII port, G751 has single SATAIII that you have to modify if you want to use it with the M2. The open side is SATA II.
Graphics remain the same.
Memory on G752 is updated to DDR4 that lifts the 32GB barrier (if you are even interested in that) and runs at higher speeds with a lower power consumption. Max memory is now at 64GB which Im about to try 64GB of DDR4 2800 Mhz. Ill post that up when Im done.
CPU performance difference is all based on what source you look at, but in the end, no matter the source any change in raw performance numbers are negligible. If you look at performance per watt, the sky-lake wins which for this platform of a high end rig really doesn't amount to a hill of beans but it will for the market going for longer battery run time. Might give you 5 extra minutes on these machines.
G752, new platform, new look even though some of the finger print smudge saving matte black still exists on the keyboard.
I was on the fence myself but not over the G751-DB72, what I was comparing was the DB73X and the G751 still lost out as I type this from a G752-DH72 that so far has had the M2 upgraded to the 512GB 950 pro and the 1TB spinner replaced with a 1TB 850 EVO. After the memory Im done until they come out with the 1TB M2 later this year.

“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein