How could it be comparable between SK/US and NK/China? SK is occupied by US, it is American's obligation and interest to develop SK as a forward base. If China has hundreds of thousands troops stationed in NK, NK would not have gone nuclear, there won't be UN sanction, and China will certainly help NK in development.I won't be surprised if China one day regrets having helped UN sanction Korea to please US.
Don't even bother to bring up "Why China did not station army in NK". No country wants foreign army on their soil especially Koreans. NK wanted PVA to leave in 1950s because NK wanted to hang on to the richer and stronger USSR and also to show their true independence to SK. It is NK's "fault" to play USSR against China, it is NK's "fault" to go nuclear after the collapse of USSR instead of turning closer to China. China has nothing to regret.
People have to pay for consequences for what they do instead of demanding other's for help without cooperation. It is good that NK has realized it now and began building closer relationship with China, it will certainly be rewarded. But blaming China for the past is certainly misplaced.