which challenge platform to use? (artstation community challenge? art war? rookies?)

finchrupert

I'm a student artist who will soon be graduating in concept art and i heard online challenges was a good way to learn and gain exposure. i work mostly in environments but do a bit of character work too. i've never taken part in a challenge before so i wanted to know what's the difference and which ones were best to join? 

or, if it depends, what does it depend on? 

i know a lot of this will be subjective but i appreciate whatever you guys can add.