I used to have an Oly E300 and the advantages it had over my Alpha 300 were:
1. Sensor cleaning system that actually works. In 2 years and many lens changes in the field not once did it ever have any much on the lens. The Sony was constantly filthy.
2. The build quality of the E300 was much better than the Sony and this may have contributed to point 1. I suspect that the Sony was shaking off bits of internal stuff onto the sensor.
OK, an E300 is not the same as an E420 but hopefully that might help.