If you weight train 1st you will burn up your muscle's glycogen stores and then when you do your cardio 2nd you will immediatly go into fat stores. This of course, if your doing low-mod level cardio. Doing cardio 1st deletes the glycogen neccesary to train heavy.
Whole point of weight traing is to build muscle. You will build muscle more effieciently if you do cardio after you workout. So just do it after.
There is really no benefit in doing cardio beforehand unless you are just doing 5 minutes to warm up.