I thought it WAS rubber after Enel couldn't electrocute him. Never thought about it again.
It's the difference between a Paramecia fruit and a Logia fruit, from what I can tell.
-Paramecia fruits (Luffy's fruit is in this category) can allow the body to take on the characteristics of a substance, but doesn't actually transform that person's body into that substance itself (For example, Magellan can manipulate and produce poison, as well as ingest it, but can't turn his body into poison; Jozu can't turn himself into diamond, but can produce an armor of it and has all of its properties)
-Logia fruits will allow the user to become that substance itself and manipulate it to a limitless extent (Crocodile, Ace, Sakazuki, Kuzan, etc.).
So again, weird distinction, and I could be wrong for all I know. I'm just going off what I've seen and read.