Since I've had a few people that like to wait until the burning -con affect wears off before they advance -

Help Stats

When you advance a level, your character is improved by the better of your natural stats or your adjusted ones. For example, if your CON is naturally at 13, but you have increased it to 25 through magical gear or spells, your character will improve as though your CON were naturally 25. Or if your WIS is naturally 20, but your adjusted WIS is 14, you improve using the 20.