Thanks! That explains it. Is there any subtle difference if I make an offer to do something in the future? For example, I want to invite a friend to meet up tomorrow. Is there any small difference in meaning between these phrases?
There's a subtle difference.
We use both in daily conversations for the same purpose, so the difference has become almost invisible.
But i'll try to explain... In Turkish, 'you' refers both sen (informal) and siz (formal).
For both case:
a) In an informal speech, ('you' refers to sen), both tenses refer to the same meaning and almost have thesame tone. But 'ister misin' is a tiny bit more polite and cool way of asking.
b) In a formal speech, ('you' refers to siz), we better prefer simple present tense (ister misin).
ex: When a secretary relays a statement of his/her boss to you, she says: "Ahmet Bey görüşmek sizinle isterler." You see? "Ahmet Bey sizinle görüşmek istiyor" sounds like 'he wants it right now and sounds unpolite.