I'd need to be the driver and do a long road trip in both to know for sure! I've only been a passenger in a C63 but tbh from that limited experience I can't differentiate which would be more comfortable for a long road trip. The C63 can adjust its side bolster just as the M3 can....
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///M3 | E90 LCI | Space Grey | Fox Red with CF Structure | 6MT | 260M | My build
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