Old Leslies are like old Hammonds; they all sound different, even if they are the same model. You can get a Leslie 122 and it might sound great or it might sound like crap. It all depends on how it was maintained, how the tubes are, how the capacitors are, how the speakers are, etc.
The nice thing about new ones is that you know what you're getting. Also, the new ones interface more easily with the new Hammonds, utilizing the 11 pin connection. With the old ones, you need to use a converter box and that just adds one more thing in between that has the potential to break. When I used my 122 with the XK System, I had one of those converters boxes (from Hammond) and it started developing problems after a year on the road, at one point having a component failure that caused the speed switching not to work.
So with all that said... I would go new. Either the 3300 or the new 21 system.