From demo and show systems I have built that run games or certain benchmarks on indefinite loops for the visuals, I have bounced the same boards between SATA3 SSD's (nicer end Samsung or Intels) as well as some NVME M.2 drives.  Can I see the difference (these were on Z170/270 boards primarily) YES, I could, boot was faster and program launch was faster, but it was **NOT** by a large margin, not at all, it was fast enough to see it, but little else.  Which to some is important, there is a difference in top speed between a Bugatti Veyron and Chiron, but it is not much different and at a level that 99.9% of owners will not see/use, but it is there, and I think currently, that is where this is.
 
One other thing to point out.  Personally, i would go with the FTW-K.  It is a bit more refined as it was a couple years between the Classified launch and the FTW-K, but while the M.2 performance may not be enough to sway you, this might be.  As the FTW-K pulls its M.2 lanes from the very bottom PCIE slot, which is largely unused by most people, it means that the PCH is not losing 4 lanes, and this is where there are extra USB headers on the FTW-K, as there are more lanes being dedicated to peripherals.  An additional USB 2.0 and 3.0 header can be a big deal for some builds, especially with VR becoming more popular. 
 
IMHO the only thing you are losing that will be missed by many people is losing the Creative audio on the classified, unless you are a competitive bencher/cruncher who needs all 4 slots.
 
Both are good boards, and I don't think there is a BAD choice there.  So, 4 way SLI or more USB and faster THEORETICAL M.2 performance?  Core3d Audio or Killer NIC?  I think that is what it really boils down to.