I've read many times that low intensity stuff like walking is the best strategy to burn fat and spare muscle. Personally, I don't do any cardio when I'm trying to gain muscle. I find it to be counterproductive since my body needs the extra calories and forgoing the cardio lets my body recover faster from lifting.
I would say do Interval training cardio where u will do no more then 20 to 30 mins... HIIT helps both for cutting and bulking.... Instead of circuit, just lift heavy and have more rest periods.. that helps me grow... and if you up for caloric intake you should be growing..