Selectors last year was amazing, doing it again this year.
I think pretty much all of the festivals at the garden have a similar relaxed vibe to them, so you should probably choose according to which ever dates \ line up suits you.
Regarding Barbarella, I went once last year, and wasn't that great, I think the fact that sunrise is much later at the end of August takes a bit from the experience (think Amnesia in september with 6AM finish time...), but I'll probably give it another try this year.
Taxi from what I remember was 20 euro, but are hard to find if you leave when the "day" program finishes, so leave a bit earlier.
Dimensions festival near Pula, on the other hand, was just terrible, so stay clear from that one...