Under ideal circumstances, you can get around 50 tons onboard a -73. An A300B4 will max out at around 45. However, the Bus has much more in the way iof volume and is capable of carrying a wider selection of ULD types. The -73 has superior range over the B4.
For an integrator doing short/medium routes, the B4 will probably be a better deal. For long hauls or dense loads, the -73 will probably be a better choice. Horses for courses and all that.
The A300-600F will lift more than the -73, albeit at slightly shorter range.
PS
The B4 also has a "proper" 3-man cockpit.
