Forms (show and process) on the same controller or on different ones?

The question is - what is the best approach 
to show froms (if post/get is empty) and process data(if post/get contains data)  in one controller or to make different controllers(or in this particular case methods), and if data don't pass the validation redirect back to previous one?

Which approach is better? 
If you do all in one controller(method) it become a mess, with lots of IF statements and curly bracers scattered all over the place☻