A few random questions on Slovenia
 

I've finalized our plans for Slovenia and Croatia (Istria) the last 2 weeks in June. My questions: does anyone know if T-Mobile gets service in these countries? (I get ambiguous answers from T-Mobile). Also, if we don't get service and/or don't have good access to GPS while driving, can anyone recommend a good driving map? Or have you found the car rental agency map is sufficient?
Lastly, preference for Postojna or Skocjan Cave? I've heard the latter is somewhat less "touristy"/crowded.
Thank you very much!

I have TMobile and it works fine in Slovenia. Istria too. It’s not difficult to drive without any map unless you are going to some random address (like a restaurant with an ambiguous address.) everything is pretty well signposted.

I have only visited Postojna, the good thing about that one is it includes Predjama castle in the ticket. The cave is super touristy but they manage the groups well, with space between each group as you move through. The castle is pretty cool and never felt too crowded to me, but maybe it will be last two weeks in June.