The horsepower of the Opel Astra J is 5 % bigger than the MG 6. While the difference is little this should not affect the car performance in a noticeable way.
The Opel Astra J has 1,093 mm longer wheelbase when compared to the MG. The shorter wheelbase, the less time exists for shock absorption by the separate suspension components. Therefore the Opel Astra J with a longer wheelbase should provide the smoothest travel. But, the MG 6 with shorter wheelbases have tighter turning angles and is able corner faster than equivalent cars having longer wheelbases.
The Opel Astra J is 8 mm longer when compared to the MG 6. The length of the car is significant in terms of driving comfort and room in the boot. The Opel probably will be offering to it's users more space for legs. At the same time the Opel will be better driven but the MG will be a bit easier for parking due to shorter length .
If you are a tall man or you tend to carry in the trunk thinks of large size, the Opel Astra J, which is by 2 % higher than the MG 6, would be better pick for you. However, these two vehicles do not differ much it the aspect of its height and it should not be much perceptible.