The main differences are the cable length and included accessories:
10m DC Extension Cable Plus：
Length: 10 m (32.8 ft)
Connectors: DC8020 male to DC8020 female
Recommended when a longer extension is needed.
DC Extension Cable for Solar Panel：
Length: 5 m (16.4 ft)
Connectors: DC8020 male to DC8020 female
Includes a DC8020-to-DC7909 adapter
In short, the 10m DC Extension Cable Plus is the longer option, while the 5m Solar Panel Extension Cable additionally includes a DC8020-to-DC7909 adapter.