The Hornby HAA is available in 2 versions, the Railroad version is around the same age as the VDA / OAA wagons, where as the main range version was retooled and much improved in 2003. Along with this new version of HAA came the HBA, CDA, and MHA all based on the same chassis.
Both companies make a HEA which if you trace there production back actually come from the same origin, however Bachmann have modernised the wagon in recent years.
Hornby make the PCA depressed tank which is a pretty good model, however Dapol and Lima made these wagons first and Hornby actually own both versions now.
Hornbys Clam, Rudd, Tope and Shark engineers wagons are actually good models, but they are recent models.
Hornby where the first to do a container flat, which is now getting on for fifty years old, however they have recently done a KFA single container wagon which is equal to the Bachmann container wagons in detail. Bachmann are due to do the original version container flat soon which should make the old Hornby container flat totally redundant.
Both companies have produced the OTA timber wagons which are both good models, however they have made different versions so that each company has a different wagon.
Hope this is of some help.