In terms of rendition of the A4's shape, treatment of detail, and paint finish, the Hornby A4 is much the best; but there are some small reservations. Different size apertures on the double chimney for a start - definitely incorrect - but easy to rectify, a tendency for some of finer detail to fall off while running. It comes with a DCC socket. Best ever of the many OO RTR offerings of this class.
Bachmann's offering is remarkably good, considering the body is based on a Trix moulding from circa 1960. It has a heavy split chassis mechanism which will pull anything you care to hang behind it, but requires significant work to convert for DCC. Less detailed, not as well finished, but very robust. If children are involved, or outdoor operation, this would be my choice.