The great thing is that they all use the same core system, so try them, or even install them at the same time (except Elementary), and use them until you find your favorite.
Why would you use? I would list why I tried it and used it for 3 months - at work. I stopped using it because it became a little difficult to manage the stability issues, build problems (at that it was very unstable as I mentioned above) and then the start-up I worked for got funded and gave everyone the then latest Macbook Pro (I had been using an Air at home anyway).
I found it pretty. Simple. Not at all cluttered - very clean look. I was used to OSX and its look and feel was/is very much inspired from OSX (or a similar UX/UI design philosophy if I may say so). And it's open source.
I kept using it also because I kind of fell in love with Midori (the Elementary browser and that's one app from Elementary I still miss on OSX).
Why not Ubuntu? Honestly, everyone else in my team used Ubuntu and it worked perfectly fine for them and whenever I ran into issues I was just asked one question "why the h not Ubuntu?". It was mostly aesthetics. Elementary felt very simple and pleasing to my eyes (yes, even when compared to X/LUbuntu) and for some reason I never really liked Ubuntu after Unity and all that happened (that was back in college - can't recall all the reasons as of now). Also, it was definitely lighter then Ubuntu.
This may not be the answers you are looking for but I thought I will chip in with my own reasons. And yes, I am going back to Linux too. My Air is 5 years old now and is already showing its age. I don't think I would like to spend the latest Macbook Pro kind of money for another Apple laptop. Then most probably I will try one of these - Elementary or Solus or Apricity etc on a lightweight 13ish inch laptop.
Ubuntu veered towards the touch stuff, using nontraditional approaches; I honestly get lost these days when I have to use it.
I prefer it over Ubuntu because the UI makes more sense to me than Ubuntu's Unity.