I'm so glad you got invited to pref at both!
Even though you have said that you have really made up your mind where you want to be, you have to keep in mind that if you pref both, there is a good chance that you could end up in the one that your heart wasn't set on. You could rank one as your number one, but if you're not high enough on their list before they fill up their quota, you could very likely go to your number two... But i'm sure you know all this, just reminding you
Good luck and have fun, pref night is so special!