You can see from this answer, they evaluate to identical types.
As for why there are two names, if you look at how the #defines are structured (again, see linked answer), a type in OpenCV has 2 parts, the depth, and the number of channels. The system is flexible enough to let you define new types with up to 512 channels. It just so happens that when you specify 1 channel, the channel component of type is set to 0 which makes the result equivalent to simply using the depth CV_8U.