If you only had time to do one, which would you choose? I can get to either, so distance is not an issue, I just don’t have time to do both.
I have visited both caves, but Postojna was too long ago to make a fair comosridon. Perhaps this information will help:
Postojna is more touristy, with colored lights. A little pseudo-train is used for part of the visit.
Skocjan is considered more natural. The overall light level was low enough that all the cave formations looked the same color to me. (I was 63 at the time of my visit.) I think there's more walking required for the Skocjan visit.
Similiar to what acraven said, Postojna is definitely bigger and more touristy. If you don't have kids and don't have huge interest in caves, just go to Škocjan since it's much smaller but makes up greater natural sights.
Also, there is a stunning view behind Škocjav Cave, but Postojna Cave features Predjama Castle not far from it.
I have only been to Postojna, and was not overwhelmed. It's a matter of personal preference, but I would have preferred architectural, cultural, or artistic highlights. But because it was a day out from a very small cruise ship, our options were limited. I certainly love a beautiful mountain view (outdoors), but a cave is just a cave to me.
I’m still torn. I notice Rick doesn’t include either on his tour of the Adriatic, kind of makes me wonder if either are even worth doing.
Oh, they're worth doing; it comes down to what the trade-offs are. I'd prefer a second day in Ljubljana; I would not prefer a second day at Lake Bled (unless I was day-tripping in the surrounding area). I assume a day with a car in the Julian Alps would be preferred by many people. Unfortunately, there's never enough time for all the places we want to go.
One thing I definitely pay attention to in cases like this is the logisitics: How much time will I spend on a train or a bus? Is the schedule frequent enough that I won't have to hang around for 90 minutes or more before starting the trip back?