Help deciding between Photoshop and Illustrator

I am learning to use both of these applications but I am having one problem. I don’t know when to stop using illustrator and finish with photoshop or vise versa.

I don’t care how big the file sizes are or anything like that, I just want to know what application should be used to make the best graphics. If photoshop or Illustrator can be used for 100% of the graphics then please let me know what to use.

I just want to be able to make professional buttons and other graphics for flash sites. Thanks for any help.