Then you must have been only at Madrid, Osaka and maybe a superzoo like Berlin or San Diego, and for sure not Barcelona. It would be the only thing that explains why you prefair Barcelona (that is disappointing in species (nothing too rare, unique and spectacular), exhibits (dry hot dirt for most, small cages for not so small primates...) and aesthetics (signage, horrible animal caricatures...) as I already said) over Madrid zoo (that is better than average zoo). Madrid have 284 taxa in Zootierliste, and Barcelona 275 (this surprises me a lot, as I always tough by my visits that Barcelona have MUCH less species). On average, animals are easier to photograph at Madrid. Of the zoos that I've visited in my life, 9 were worst than Madrid (including Barcelona zoo), 8 were better than Madrid, and the rest were more or less the same quality than Madrid.
If that's just a question of Barcelona having improved very much since my last visit, and now Barcelona is much better than then, I will eat my words... certainly my last Madrid visit was much more recent.