GNU Compiler Collection or GCC is free and have been around for a very long time. GCC is supported bu volunteers. This means there is a big community where you can find help for free and from different sources – but response can take longer or you want get at all.
But there is several significant advantages of GCC compiler collection:
GCC is available for many different processors like x86, AVR, ARM;
Code is very mature and works well. It is also well documented.
GCC and AVR-GCC 
