Probably not far off from the AK durability wise. But far, far more precise. The HK is good to 800 meters, something an AK was never built for.
The reason the AK is so reliable if you fill it up with mud is because its lose as can be. But that also causes it to suck as a rifle, you can't hit anything. I don't know I'm still not a fan of Soviet block guns. I almost bought a Dragonuv insted of the Sig, but the quality isn't even close. The SIG OTOH uses a highly refined AK bolt, with a FN Fal gas piston system, so it should do pretty well. Not sure what the HK uses, I'd guess its like a G3.
These things are pretty pricey these days so I plan on keeping them nice and clean. I can't see throwing a $2k gun into the mud.