I don't know about you guys, but a six cylinder in a W126 (and even a W116) does nothing for me.
I once had a 280SE and it bored me so much.
It depends which one and what you want to do with it.
For example, a 300SE is actually quite a bit faster around town than an Aussie 420SEL. But on the freeway a 420SEL is a better cruiser. 420 has really tall gearing. Drive a 300 and a 420 back to back and you'll be surprised. The late imported 420 is another story though. Obviously a 560 will blow both out of the water, particularly an private import.
The aussie 380 has a 3.27 rear end so it is much quicker. (vs a 2.47 on a 420 and a 3.46 on a 300)
The M110 suffered a lot from emissions, so any Aussie delivered car from 1976-1985 will be quite slow. But the import cars are rather lively - if you can find a south african car that hasn't been driven into the ground.