It is when your site becomes a successful higher traffic site that the big differences emerge. These plans sound so great until you are committed. Then you realize they just do not work. Here is the problem. Bandwidth is expensive.
Somebody has to pay.
The flat rate plans make money by selling the same bandwidth to dozens of other people too. They don't care if they actually deliver content because they get the same amount of money whether they do or not. The result is slow performance. To counteract the effects of overselling they have to get rid of high traffic sites. If your site draws too much bandwidth they find some contrived reason to kick you off.