I've not eaten at the Knightsbridge restaurant, but have been to Seven Dials once, Air Street and Guildhall a fair bit and Spitalfields loads, and I'd always recommend Spitalfields. It's more intimate and down to earth.
Guildhall is big and fun, but very City, whereas Air Street is simply loud, very loud and gaudy. Plus I've only had one good meal in Air Street, the rest have been average compared to their other locations. Seven Dials was decent though, however I'd recommend Spitalfields.