if you have the $$ u're better off doing both. u're there, your relaxed and set up, and u spent the first day getting things down so you're ready to rail the second.
personally, i don't do 2 days at places i know already pretty well. it gets to be a waste of tires and money to just do stupid laps. i start to screw around and see how many times i can spook my friends and stuff but looking at them in the corners as they try and keep up.
but... if you have the cash try and do the 2 day everynow and then. but stay ahead of it by drinking water days prior and doing a lot of working out IN the heat to get your body prepared to handle the heat and retaining more water. you need a clear head and no heat related issues so you an focus on passing that schmuck who just won't get out of the way.