When to use "me dices" or "dime"

Question

If you have more questions, tell me.

Would it be

Por si tienes mas preguntas, me dices

or

Por si tienes mas preguntas, dime

Answer 1

In Spain, at least, we would not use por in these sentences, and we would modify the last one slightly:

Si tienes más preguntas, me dices.

Si tienes más preguntas, dímelo.

The second one can be used in any case, while the first one is quite informal and can sound awkward in some contexts. In case of doubt, you are better off using dímelo, which, as I said, is fine in any context. Assuming, of course, that it is relaxed enough to use tú; if you are using usted, it becomes:

Si tiene más preguntas, dígamelo.

In this case, the first version is too informal and has no equivalent.

Answer 2

"Me dices" suggests to me that there's a gap between the request and the decision to proceed. When you say "dime," you mean either "now" or "please do not keep it." "Por si" means "Por si acaso"(Just in case you have any questions, just in case if you decide to do so, he/she doesn't think that you'll have any..)

I'd say:

• Si tienes más preguntas, me dices.
• Si tienes más preguntas, dime no más.
• Si tienes más preguntas, no dudes en consultar(me)
• Por si tienes alguna pregunta, acá/aquí voy a estar(para responderte, para responderlas).
• En el caso de que tengas alguna pregunta, voy a andar por acá(para ayudarte)

EDIT:

• Me dices ==> you'll tell me, I just want you to tell me, I wish you'd tell me, let me know.

Doctor: Si te duele, me dices, ¿de acuerdo?

Doctor: Si le duele, me dice, ¿de acuerdo? (with more formality)