The AK-74SU has a vertically-folding shoulder plate, whereas the AKS-74U features a horizontally-folding buttstock.
The AK-74SU comes in a more grey-ish colour, and is primarily made of metal components. The AKS-74U has a wooden hand hold underneath/behind the barrel, and is wholly darker in colour.
The AK-74SU is primarily used by Spetsnaz special forces, whereas the AKS-74U is used more broadly with airbourne units, or in other situations requiring high mobility with little space.
The AK-74SU does not appear to have as high a firing rate as the AKS-74U.