In terms of meaning, you're communicating the same thing, and they can be used interchangeably. I'll give you two examples. A friend comes in to show you his/her new game record, the highest in the league and among all your friends. He/she brags about it, then you say "Don't get too cocky. I'll beat your record soon". Take the same circumstance, but your friend picks on someone in particular, talking about how bad their opponent in the game is. You can say "Don't be cocky". You could also say "Don't get too cocky". To conclude, they tell the same thing. You could use one instead of the other. It's just a matter of the timeline