Everything I know from my years of training as a personal trainer, and in my own life experience is that cardio is best done after weights if they have to be done together. You benefit from already having used up quite a bit of blood sugar, so you are more likely to partition from fat for energy at that point. With HIIT training either taking in some BCAAs before or just after would be beneficial in avoiding a catabolic state. Really you should treat a HIIT session almost like a weight training session as the stress on the muscles and CNS are easily equal to a weight training session. You have to get some food in there to keep the cortisol from eating you up. I wouldn't eat a big post workout meal if you did only cardio but if you are lifting to go for the gusto, get your nutrients in immediately your body definitely needs them. All things being said the sooner in the day you elevate your metabolism via cardio or weight lifting, the longer it is elevated through out the day ='s more calories burnt, so early cardio / exercise is most beneficial in weight loss efforts. I have read a lot of different studies on the subject and almost all forms of cardio end up equalling out at the end of the day as far as fat burning goes. The big difference is the amount of time spent on the actuall work being done. HIIT provides more cardiovascular benefits, and can improve your lipid profiles, improves the bodies ability to clear lactic acid and is great if you do any type of explosive competitive sports. Steady state aerobics take longer and bore the crap out of me unless I want to do some reading.
HIIT is definitely my favorite, as it allows me to push myself further and harder. Every interval is a new test of my will to finish. It also strengthens your heart quite a bit and radically increases your MaxVo2 (amount of oxygen your blood can carry to the muscles) as well as saving me an extra 10-20 minutes of my day for other things.
