Neither! There is nothing 'correct' about being lazy. 'Pls' is not a word. Nor is 'plz'. Or 'u' or 'ur'. Or 'cos' or 'coz'. And don't get me started on 'wanna' and 'gonna'. Maggie, please...yes, and I mean 'p-l-e-a-s-e'... it took me all of two seconds to write those six letters and it really isn't a problem...understand something important: Nobody will take you seriously if you don't write words properly. Using these silly, gimmicky shortcuts will make your writing look lazy, childish and unintelligent. If that's what you want - fine. Otherwise just write normal English.

Neither is "correct", if you mean "acceptable in standard English". Just write normally and use "please", please.

Does it take so much extra effort to write some vowels? Look, I've just given you two or three dozen myself!

both are shortcuts for saying please, so there isn't a correct one. Although, many others use "plz" instead of "pls." Either one is fine though.

I would recommend Pls. Simply because there is no z in the word please. Makes more sense to me. ☺