I am not actually sure what the difference is, maybe the Material? I just know the main difference is how much UV they put out and how fast it puts it out.
I wish I had saved an email be and a friend had a year or two back, he had a UV meter and tested all of his bulbs ( He was a bearded dragon breeder, and this helped him know when his MVB bulbs reached the end of their life so he did not have to stay to the normal one year and throw some out earlier than he needed to)
With every MVB bulb you get, you are supposed to let it have an hour burn out period when you first get it, because in the first out it pours out way to much UVB, and then starts giving out its normal amount. Essentially he said Compact bulbs keep pouring out UVB for the first month like that, some actually not even being able to produce hardly any UVB after that.
You would be heart broken to see a Lizard or a Turtle with PKC, their eyes are swollen, they are lethargic, and hardly eat. Very luckily the healing process starts as soon as the bulb is removed. One tortoise I had was 100% better after just a week with no medical attention, and he was a pretty severe case.
He was actually half way healed by the time I took this video, before he looked half dead.
YouTube - New tortoise