Just a conversation starter, to reflect on how you got into Codeigniter, What sort of projects you have worked in.
Let me start;
I used to customize Wordpress and PHP Scripts before, and used to work on pure PHP. Then slowly I understood the need for a framework, something that can give me the basics to get started fast. I tried a bunch of frameworks at the time, CI was the easiest to get started and started adopting it to my workflow.
I got involved when CI was in its late 1.x releases and that transition to 2.x
The ones who are new to CI now are probably going through the same, wondering if you should use the 2.x stable or 3.x dev
Well If you are just getting started go with 3.x you can always swap that System folder once its stable. Get used to the MVC and that pattern of thinking.