I was there last week in a landy.
Southern approach road is bad gravel/mud and lots of deep sandy ruts. Not easy on a bike, fine for the landy - 3rd gear all the way for 120 odd km's(??). Then the last 45km is a sandy rutted track through bush (ie; not a road, just a track). Ruts are not so big, and smooth, but less fun on a bike.
You would be better to go via the north route (turning just after the vet checkpoint when you come south. This is mostly a gravel track, not great, but better than in the south.
Just before the border turn directly south, camp in the Aha hills by the south sinkhole (nice track to follow a few km if you have tracks for Africa etc) and then down to Xai Xai and then follow the signs for the cave. The track is rutted and sandy, but firm sand as per the last 45km coming from the other direction.
Not much out there by the way! But cheap living and the villagers are really friendly. Xai Xai needed a football if your felling generous ;-p