NCYGS is simpler to achieve mainly because of one reason: you have three opportunities to achieve it (from RG to AO, from Wim to RG, and from UO to Wim), while there is only one opportunity for CYGS. Whether one of these four possibilities is somewhat easier individually than the other three doesn't matter compared to this factor of 3:1.
I don't think there is a significant difference in any of these possibilities. The only way one of them might possibly be easier is if the Australian Open was a player's toughest slam. Because it's the first tournament of the year, you could spend the whole offseason mentally preparing and training for it. The other slams all have Masters 1000's and notable 500's tournaments before them, so you can't exclusively focus only on the upcoming slam.