Large Rex: Med vs Soft firmness for knotting? Preferences?

So I posted in another thread that my goal is to hilt a large Rex. I have a BEAUTIFUL boy but he's in medium firmness and honestly feels pretty hard due to all the thickness of course.

From more experienced toyers who have actually hilted this bad boy, would you prefer a medium or a soft firmness?

My problem right now is getting past the first sphincter with rex or any other toy, really. Would a soft aid in squishing in there? or is the firmness of the medium better to 'straighten' it out?