It's a hard one to call as it depends on how many users you are expecting to use your site at any one time. The main advantage is a fixed cost per month. The disadvantages are your site would be slow if a lot of users using your site at the same time and the cost of upgrading your site into the next bandwidth is usally high.
Have a look at dialtone internet http://dialtoneinternet.com
as they provide 50 GB of bandwidth and allow 1 GIG increments for only $3 per month.
Others require you to purchase in blocks e.g 50GB at $150 so be careful!!!