Black City, because it's offering of rare items is ultimately more useful.
Catching pokemon is fine and dandy, but the thing is...I've...got all of them on previous games anyway. And with the Black City trainer battles, that allows me to simply GTS a bunch of fully evolved monsters who I could use for eggs :/
I like Black City better. I'm more of a trainer than a collector. I always just want really strong pokemon. I mainly focus on battling rather than the pokedex. That's my only regret about getting White. There's not really a great way to train your pokemon. Don't get me wrong, I still like White Forest. I like it because I can catch old pokemon and level them up for battles. I still need to find that damn Elekid.
Black City, by far. The elimination of the 6-figure wallet makes Black City's potential finances all the more beneficial, and getting the overpriced items is far easier when you have trainers you can rebattle (especially with the Rich Boy, Lady, and Vet on the field; I have both the Lady and the Veteran, which is an extra 20,000 with the amulet coin each day, on top of the other trainers and the Boss's 10,000 for beatting the approx. trainers).
In my opinion, Black City.
Even though.. I don't really have an opinion of Black City or White Forest. Black City for me because I get to train all my pokemon with all these trainers and it makes me laugh that prices they have at the stores. White forest is good too because I like catching pokemon. For White Forest, whats funny is that whenever I go in the bushes, I encounter a pokemon and I throw the pokeball at the level 5, and it doesn't catch it! it sort of amazes me.
Black City. Considering BW's pitiful post-Elite Four game, more trainers to battle are needed. Plus, being able to actually buy stones?
White Forest would be useful if it weren't such a chore to Entralink.
I have Black, and I like the Black City, but I'm a bit confused...and disappointed. I don't really think the trainers are that tough? The only ones I have (about 10) have Pokemon ranging from 20-50 at the highest. Really not that great for training (so I just do the Elite Four over and over, or Cynthia or Cheren).
Do you have to exchange over the wifi thing with someone with White to get stronger trainers?
Anyway, as of now I voted for White Forest because I like it's peaceful design and some of the cool Pokemon you can catch (porygon). But I might change my mind.