The SKS is imo a bit of a cleverer design, basically “be lighter especially further out, and therefore don’t need to be as strong”, and much less ugly imo but I don’t think there’s much in it really, both are still mostly about how well it fits to your specific bike. SKS has less coverage but is pretty long, again might be better or worse, I’ve always felt even the small mudhugger is pretty damn big, but that again depends tons on the shape and angle of the stays. If there was a smaller version of the SKS I’d have been pretty happy with that but for my bikes it was really long and that’s not just about looks, it creates more weight and more movement and doesn’t really do anything.
Another option if you’ve got reasonably small seatstays might be the Win Wing- they don’t do an official mtb one but I’ve had one on a couple of 29ers and it’s been impressively effective. Far less coverage but it does like 90% of the job with about 30% of the size, and it’s incredibly quick to fit. it feels flexy but the actual mudguard weighs nowt so it doesn’t want to flap. Completely useless on many mtbs I’m sure and I reckon that’s why there’s no mtb version, teh design just isn’t that adaptible and would never be universal. It’s the product that stayed on my bikes.