When I was a young schoolboy (a few years ago.....) I was taught that if you ask a question about 3 or more people (or things) you ask which is the - oldest, prettiest, biggest, richest, fattest, etc. When there are only 2 people or things, it is correct to ask which is the - older, prettier, bigger, richer, fatter, etc. Modern usage in the SMS age may have changed this stipulation......

Comparative degree is necessary when comparing only two people or things. It is therefore grammatically correct to ask "which one of you two is older?" It is not correct to ask "which one of you two is the oldest?"

They are both grammatically correct. They both say and mean the same thing.