Essentially, short of the more expensive C63 Black Series this is the highest performance C63 one can buy. On paper, the C63 has the power advantage over the M4's 425 horses and the torque advantage at 450 lb-ft to the M4's 406 lb-ft. The C63 is heavier by about 427 pounds which is significant. This M4 features a dual clutch transmission while the C63 is an MCT 7-speed auto (not a dual clutch as Autocar states).
We have a basic idea as to how Autocar feels about the new M3/M4 considering previous statements about the F80 M3 and criticism of the new turbocharged engine. Autocar likes the M156 under the hood of the C63. They really like it and who can blame them? A big 6.2 liter V8 revving to 7200 rpm is just straight German muscle from AMG. We will not see a motor like this again, at least from the Germans.
The M4 feels lighter on its feet which is no surprise. It is much lighter. The C63 is said to feel old fashioned. The differences in the motors of course highlights this the most. One is the result of ecological pressure and cutting costs and the other is just raw V8 muscle. Which do you prefer? The reviewer states the C63 engine feels like it is from another era but has GM not proved ecological technology can be blended successfully with a large V8? The statements about being from another era are not fair as if GM can build a 6.2 liter V8 passing today's emissions standards Mercedes certainly can as well.
Now as far as this being a comparison test, it isn't. It's a subject comparison. Where are the numbers? Where is the race between the two? Where are the laptimes? We get some quoted 0-60 figures but considering how much 0-60 varies based on available traction this is essentially worthless. This is not a comparison test, this is the reviewer sitting there and telling us a story.
Autocar, give us numbers. Give us a real test. Calling this a head to head comparison insults our intelligence. This is a preview of a test if anything.
The 'review' concludes with the C63 being the car the reviewer would choose between the two. His reasoning as to why is sound. It offers that extra response and raw element that the new M3/M4 simply can not match. No amount of improvement in the M3's emissions will change that.