Like another poster said MB per second is like renting a car - if you go 75 miles an hour they charge you more than if you went 20 miles an hour.
GB / Total Transfer is billed based on the total amount of traffic going to your server/web sites. Or, as in our example, the total distance traveled.
Most ISPs use 95th percentile billing for traffic. You can read more about it at
http://www.seanadams.com/95/ -- basically, you're charged for the amount of traffic you use 95 percent of the time. This is actually a very fare number - it removes spikes and tells you how much actual traffic your website/server are experiencing. For anything over 1-3 mb/s, this is the most effecient, cheapest bandwidth monitoring.
Small web hosts will often offer billing per gigabyte transferred. This is useful for very small web sites where you're only expecting a few gigs of transfer a month. The cost of per gigabyte is often fixed and you pay for all that you use.