Hmm, tough choice! Dvur of course has those tropical halls (one walkthrough, one with spacious aviaries for parrots and hornbills), a few nicely-stocked aviaries for turacos and passerines next to those, and three large aviaries for water birds outdoors.
Ostrava is less bird-focused I'd say, but does have a lot of very nicely designed standalone aviaries throughout the zoo, many of them for raptors, but also ones in/next to the many pavilions throughout the zoo, and a nice New Guinean walkthrough aviary with some parrots, lapwings and honeyeaters. The main avian highlight is probably an entire area devoted to smaller Eurasian birds, with nice walkthroughs for many starling, bulbul and pheasant species, and even choughs.
I'm currently placing my vote with Ostrava, but hoping zootierliste pops back online before voting closes, so I can hopefully refresh my memory a bit better with species lists.
For now I'd say ostrava wins, because even though they lack a big highlight like Dvur's tropical house, they make up for it in enclosure quality of small standalone aviaries throughout the zoo.