I've used both 2.4x rw and 4x rw media, but I can't say I've noticed any real difference. The 2.4x is cheaper and I've not had only 5 bad disks out of 300. So what is the difference?
The difference is in the speed it can burn at. 2.4 is a little slower than 4x. Quality of the burnt disc would be the same, or close to the same.