I don't really see much of an upgrade if lets say i got a D300 as my next camera in the near future. So thats why i would go FX. Its like if you have a car that's a 1.6L, your next car if you consider an upgrade would be like a 2.0L right

But thats just me so yea...
Bigger sensor = Bigger surface area = More light = Better ISO = Lower noise = Better night shoots. (ok just taking crap here)
I don't actually plan on selling the S5 because it does its job well. Has excellent colors. And there for it will be my spare cam when i do upgrade.
The other reason is because of my lenses. The 17 - 50 will remain on the S5 but the tokina 28 - 70 would be just right on a FX. Even the voigtlander would have almost similar to the range of 35mm. Its the only way to bring out the full potential of the lenses.