Hi there, I would push you to go for either the FIT or the Sunday for one of two reasons.
Fit is one the well known bimxing companies and they have a number years behind the business so they know how to do good bikes.
Smae with Sunday. They're built to last, relaible and they're main products, like the frame, forks and handlebars, come with a lifetime warranty.
I feel with United, they are a fairly moderate new company lacking the craftmanships in the frames etc.
From a bmxing background, I can tell you that own a Fit bike myself, they are light, rubust, and from a riding perspective, they 'feel good', as in nice to ride, piece tricks up easy on. With the sunday, they weigh a little more because that's down to the Rent resistant down tubes on all their frames. Mkaing them tough to crack or dent, which can happen on regular frames.
Picking the FIT or the Sunday would'n't be a bad thing at all.
Hope this helps.
Robski (BMX Expert)