Hard to say really, they're both made by the same company. I've ridden a few echos, but no zoo. The echos do feel really nice on the rear wheel, but they're kinda short :hs:. You can get the urban with either V or hs33 mounts. if you're planning on running an hs33, I would definitely get the 4bolt mounts. If you want to ride Vs, you should definitely go with the V mounts. Adapters are available either way (4 bolt -> V & V -> HS33), though the 4bolt -> V are harder to find.
the echo urban is just an ES3, now if you were getting a pure, that would be a good echo. but for trials specific grometry, the zoo piranha that i have is 10X better than the any echo ive ever ridden, and a lot cheaper. i dont know what the boa geometry is.
get a woodman T-pro if you want a real frameand you're on a budget. It's the best riding bike out there besides the Koxx, and it's extremely close. I only got the Koxx because i knew i wouldn't break it. You are really cheating yourself by not getting a Woodman, Koxx, or Echo Pure.