Use of 'os gweli di'n dda' or 'os gwelwch chi'n dda'
When would you use the above two phrases to say please? When a single word (plîs) is available?
What is the literal meaning of the two phrases?
There's also Os gwelwch yn dda, which is what we're taught to use in school in the south. Plîs is a bit of an anglicism I guess, and whilst I've never spoken Welsh to Welsh speakers (well, outside of facebook messenger/texts or whatever), when I hear "please" being used at the train station or wherever, it's of the os gwelwch variety.
I'm a bit confused about which of them is "correct" - I had always thought it was os gwelwch yn dda, but in a lot of places (including here) I see the os gwelwch chi'n dda construction instead. Is gwelwch the imperative, in which case the pronoun is optional? Which is used more commonly in speech?