You can reach a big sucess in mobile applications industry through different ways. You can order the marketing campaign in marketing company or you can do it indepentendly. You have to understand that marketing company can make it better than you can. Meanwhile, you can learn how to make, but it requires a lot of time and efforts. If you don't want to spend you time on this, you can entrust this case to the professionals.
The future or your app depens on it's promotion. Also it depends your money and your wish.