My bikes have one of each mentioned in the thread title:
Barcons on my Bike Friday
Campy ERGO on my Bianchi
Downtube shifters on my Pog
...and the Rohloff twist grip mounted to the stem on my Mercian
If I get used to riding any one of these bikes, I sometimes have a reflexive patterned response when riding another to use the shifting I've gotten used to. For a good 2 days last summer, I kept making phantom motions with my hands towards the ends of my handlebars on my Mercian because I got used to riding the Friday. That was weird. I kept saying to myself, "OK, stop doing that," but it didn't help much.
But I don't really have a preference, except to say that I like the downtube way the least and the Rohloff probably the best, but only because I like the Rohloff so much. They all have advantages.