What reflects UV radiation more: stratus clouds or cumulus clouds?
According to the below albedo rating diagram (percentage is reflection of incident radiation) cumulus cloud is the greater total spectrum reflector so it will reflect more UV that equivalent stratus formations.



Do note that water vapour does however absorb the vast majority of UV (≈100nm) that it intercepts as we can see in this diagram:

So overall cloud UV reflectance will be quite low.