I think it's better to do a little bit of exercise frequently, then to do heavy exercise infrequently. I think that when you do heavy exercise infrequently, your body isn't used to it so you feel sore and tired afterwards, which turns you off from doing more exercise. Also, your metabolism isn't adjusted to that level of activity, so you end up eating more.
If you exercise 20 minutes a day, 6 days a week, for example, you will still have exercised 2 hours that week, and exercising will feel natural to you, not something you have to force yourself to do. It will also be a part of your daily routine, so you won't feel that exercising is causing you to miss out on doing something else. I try to exercise 20-40 minutes in the morning about 5 days a week (videos), so it's just part of my normal morning routine like showering and having breakfast.
I prefer to exercise at home because if I only have time to exercise for 10 minutes, at least I'll have exercised for 10 minutes. Whereas if I went to a gym, I would probably think it's not worth the bother of travelling if I'm not going to spend at least an hour or so, so I wouldn't exercise at all.