Both speeds are really fast for a home user.. If all you do is surf the net and read email, you won't notice any difference as web pages will load equally as fast. Even viewing videos online won't display much difference since both speeds should allow for the full uninterrupted stream to smoothly download.
On the other hand, if you constantly download large files, or download multiple large files at the same time, such as music or such, you'll notice a difference in the time it takes to download the file. Of course, much depends on the "sender" of those downloads as well.. Some sites aren't able to "send" the files as fast as you can receive them... so sometimes, you won't be able to utilize all of the bandwidth of either speed.
So the "impact" will vary depending on what you do with all that bandwidth.
Hope this helps.
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?