The Mk 2 has better overload performance and lower distortion, but both have good overload and transient response and both have mic transformers.
Personally, I prefer the Mk 2 , but frankly, there’s nothing in it.
I use an old 828S (Mk 2) as a monitor/mixdown mixer….. I removed the old internal power supply and drive it from an external switch-mode supply so I have zero hum and the noise is well down to modern digital levels; and I have that wonderful analogue sound.
I have to recommend the Mk 2 if you can find one.
It makes me wonder sometimes if there is sufficient market to make the 828S again… 30 years on…. but I doubt it.