Mostly the difference is it cost more money. I assume you are talking dsl,cable etc. This is all shared bandwidth and the rates are maximum burst rates. Nobody really gets those speeds. Even for dedicated corporate connection where the bandwidth is contractual guaranteed there are many issues
The other primary limitation is the distance to a site which normally a ping give you a good representation. It measures all network delays but most are related to distance.
A rough method to calculate the maximum possible speed you can transfer is to ping he site and then divide 512/ping time.
This will give you megabits per seconds and is related to what is called the window size which limits the amount of traffic that can be sent a time.
So if you have a 150ms ping time you get 3.4m/sec to use 15m you would need a ping time of 35ms and to use 25m you would need 20ms.
You normally don't see low delays like this except in private network.
The only way to get more is run a multiple transfers at the same time or be running multiple computers behind it. Still even then this relates to large transfers. Web pages have lots of very tiny items in them which causes all the overhead delays to not allow you to use the bandwidth.
I suspect most users will not see much difference for anything above 5m.
Want to quantify difference between a 15Mbps and a 25Mbps connection in real terms. I guess 25Mbps is in theory 66% faster than 15Mbps -- but what will that really mean to me on a day-to-day basis?