a Noveske N4 Light Shorty Low Profile Upper and the Noveske N4 Stainless 10.5 CQB 5.56mm Upper?

Is it as simple as one has a stainless barrel and the other has a black barrel?

The N4 light is a different barrel all together.  It's a chrome lined hammer forged barrel with standard rifling.  It has aprox twice the thickness of normal chrome lined barrel and is made to the same standards as the SAW barrels if I remember correctly.  The CQB is a stainless barrel, medium profile (under rail) has the proprietary Noveske "Improved" polygonal rifling and is heavier than the N4 barrel.  

N4 from website:
10.5" Cold Hammer Forged Barrel, 1 in 7" twist, 5.56mm
Made of Mil Spec M249 Machine Gun barrel steel, with heavy M249 Chrome Lining,
(appx. 2 times as thick as an M4 or M16)

Stainless from website:
10.5" CQB stainless barrel, 1 in 7" twist

To find out about the differences between the N4 barrel and stainless barrel this is a great article where John Noveske himself explains it.

John Noveske interview

The stainless will most likely be a hair more accurate and the N4 will be last a little longer.