Other options: Can I say something first? Can I tell you something first? Before you say that, can I say something? Before you say that, can I tell you something?

They are both correct but neither feels quite natural, mainly because you hae repeated the word "something" in the same sentence. I would suggest rephrasing them to avoid the repetition - for example: "Can I ... first" or "Can I ... before you". The difference between "say something" and "tell you something" depends on what you plan to say: "Tell you something" implies that you have to convey some information that the other person needs to be aware of, whereas "Say something is more general and could be used if you just want to air an opinion or make some personal statement, or recite a poem(!). If in doubt, "Say something" is more likely to fit because it is less specific.

Can I say something before you say anything? Can I tell you something before you say anything?