Both are good. It’s great that working out has become important enough in your life that this is a concern for you. You can’t really “bank” workouts so trying to make up for a missed workout isn’t a good strategy.

Your fitness goals can factor into this so it is important to understand them. If you just want to burn calories, more time doing cardio means burning more calories. Less time means fewer calories burned but either way, you burn calories! If you are training for Endurance (like a marathon perhaps) then longer, moderate intensity workouts are important. However, one thing is always true – working out for a short time is better than not at all.

Regardless of how long you workout at a session, you will benefit from what you do and the cumulative effects of your training.  When you find you have only a little time to work out, do a quick set of weights or some intense cardio for a short amount of time. It’s amazing how much you can really do in a short amount of time if you stay focused. Some fitness clubs offer classes that are 30 to 45 minutes for just that reason.  If you find you are too stressed to take the time to work out, take a day off.  Recovery is an important part of any fitness routine.  If you find you have more time to devote to working out, take the time to do both cardio and weight training or even an extended session of one or the other.  There are many ways to freshen up or mix up your routine.  Talking to a personal trainer and having them help you devise both long and short routines will also help to keep you motivated no matter what your time frame is. What is most important is that you are active in some way every day, even if it is just out to walk the dog or do errands.