The main difference between the 428 and the 828 is the number of channels that each unit has. As their names may suggest, the 428 is a high quality four-channel microphone preamplifier, and the ISA 828 is a high quality eight-channel microphone preamplifier. Both units can be used to record microphone, line-level or instrument sources.
There are also a couple of other differences between the 2 units: On the 828, variable impedance switching has now been implemented on the Instrument Input, as well as on the Microphone Input. Also, the VU Metering from the 428 has been abandoned in favour of more compact LED metering, so that the unit still only takes up the same 2U of rack space that the 428 does.