It's purely personal preference but more than that its what you want more emphasis on. Typically the best way is to strength train first then go get your cardio in. This is because if you do cardio first you'll deplete your glycogen stores then you'll have no energy for weights. However if cardio is more your emphasis and the weight lifting isn't your main priority then you might be ok with this. Scientifically speaking you need the glycogen stores for heavy lifting. After you deplete the glycogen lifting heavy then you do the cardio and run on fat so its supposedly a great fat burner at that point. With that being said those who are seeking weight loss, cardio should come first, but if gaining muscle mass is the goal, it's time to hit the weight room first. Exhausting oneself with a big run right before weights and resistance training doesn't just up the risk of injury, it also means you'll have less energy to throw into a really good weight training session. If your goal is strictly a lean body -- not to be strongest, or most powerful, but achieve maximal leanness -- I always recommend implementing high-intensity interval training at the beginning of each workout. that doesn't mean weights aren't important for fat loss. In fact, when it comes to analyzing the percentage weight loss that's comprised of fat versus lean tissue like muscle, weights have cardio beat overall. It's important to get the opinion of a doctor and certified personal trainer or exercise professional before proceeding, but the takeaway is simple: if you're exercising, no matter the order, you aren't doing anything wrong. Depending on your goal, you may want to choose one type of exercise over the other. Of course, you could alternate days and avoid the question entirely.
