The Bachmann 66 is about 10 years newer, a lot better detailed and a more powerful mechanisim. The Hornby one is the ex Lima model and has a nice paint job but if you can afford the Bachmann one I would leave this one.
Both models have the front skirt as a separate piece that is a push fit, but you can't fit it and the couplings on the Bachmann model as it is fixed to the body rather than the bogie like the Hornby one. Air pipes are supplied but thay can interfere with the coupling as the bogie swings so are really more for a display model than one that's run a lot.
Both can be run staright from the box without adding any details and they still look good.
Other prototypical trains for the 66
Freightliner
Bachmann produce the twin freightliner flats for the 66 ,
Bachmann produce the Heavy haul bogie coal wagons.
EWS
Bachmann produce bogie 100ton tanks
Bachmann & Hornby produce 4 wheel TTA tanks, (sometimes run as only 4 or 5 wagon short trains).
Bachmann & Hornby produce 4 wheel engineers wagons, MTA, MHA etc.
Bachmann & Hornby make the seacow and sealion bogie hoppers again seen in trains of 4 or more.
Bachmann make the bogie JHA coal hoppers.
(I fitted kadee automatic couplings to my bachmann 66's but this involved cutting away part of the skirt to allow them to swing with the bogie).