Your question cannot be easily answered.
Most of the time, the speeds that are advertised, cannot be attained by one of several bottle-necks.
For example, Verizon FiOS, offers speeds of up to 50MBS download BUT, the ability to actually download at those speeds in highly unlikely.
Why? Because of throttling. The amount of data allowed to be streamed from a server, is usually controlled by the server admin, to prevent DOS (denial of service). With enough "requests" for data to a certain website, the server would be overloaded and then shut down for a while until demand can be normalized.
Try this test somewhere where you know the speeds being offered. Then go to a site like Microsoft. Download some piece of software trial. The very fastest you will stream data will be around 2-3MBPS.
Now, there are other factors. Your computer/router/network setup can also be a factor, but any/all of them would likely slow down rather than add speed to your perceived/desired results..
In short, any internet service that offers at least 5mbs down and 2-3 up, you are fine, and won't notice much improvement by buying more.
One area where speed may seem valuable would be anyone who has several online "gamers" in the household. IN this case, more speed would be helpful.
Then again, most any router set up would be able to control and deliver bandwidth as needed.