The difference between the two is the counterweight not the backingplate. I believe the 7336 actually comes with a backingplate but you won't be using that one. It is a rigid plate which is no good for detailing, only *sanding*.
Basically, you will be starting off with nothing, just the machine. You will have to buy a backingplate and pads to go with that machine. If you buy the Sonus kit, it comes with everything you need to make both machines work: backingplate, pads, etc.
The only thing that will be different is the fact that the 7336 comes with a 6" counterweight which was designed to go with a 6" backinplate, and you will be using a 5" backingplate from Sonus. You can either leave it alone or buy a 5" counterweight from Coastal Tool online. I think I paid like $2 plus shipping for it.