I own neither, but from reading a good deal about them, the conclusion I have drawn is that even if the higher wattage of the 28BSST2 compared to the 7BSST2 is not needed for a particular speaker/room/dB level, the 28BSST2 has a sonic advantage due to:
1. approximately twice the watts in Class A mode before going to Class A/B
2. a larger power transformer (and power supply capacitance?)
Whether this is worth $9000 more ($19,200 versus $10,200) can only be answered by those that can afford it. My personal budget is $2K, so I won't likely even be seeing a 4BSST2 for some years to come until a used one is down to that price!
Steve