I'm a wagon lover so RS6/E63S will always be my choice but of the two, I'd go M5 over M8GC. For the looks, I think the M8 looks pretty chunky and while the M5 looks boring it seems more "svelt" due to the harder/crisp body-lines compared to the M8's more curved and soft body-lines which make it look a bit more portly. The M6GC design is honestly still one of the most beautiful modern BMW's to date, nothing has topped it in my opinion.
Interior on both are great in materials and pretty lackluster in design.
I haven't driven the GC yet (only driven the coupe), but to be honest I don't think the differences will be that large or noticeable in the handling/dynamics to warrant one over the other, especially based on price (more on that below). Also do know they both have ZF transmissions compared to your DCT (or manual). Some like myself don't like the ZF, some do, so you'll have to drive it. The M5 is lighter than the F10 but honestly, it feels its size and weight, and I'd imagine the longer wheelbase of the M8GC will add to that feeling.
The biggest issue i have with the M8 is the price. I loaded up two equivalent spec'd M8GC and M5 (both comp pack) and the difference is $25k, and personally I don't think the M8 gives you $25k worth of extra equipment or experience. I also like sunroofs on my M cars (another unpopular opinion) and no M8 has that option, and weirdly massaging seats are a missing option on the M8. With the M5 discounts are already pretty easy to get, so the M8 would need significant discounts to be worth it in my opinion and I'm pretty sure the coupe and vert receive bigger discounts than the GC. I know these are $100k+ cars, but still, $25k is $25k, hell that's a Miata right there.
Good luck with the decision OP.
