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WiFi has a fast  connection and you could stay  connected as long there will be a connection, its a good way to downloads music, images, videos and the connection is also faster to download files with WiFi connection

If you go somewhere but far form your house you will not have a connection  and you can find connection for free it some places.

3G is a phone service that you can browse stuff at anytime where ever you are, its not that faster as WiFi so if you wanna open something it will be slower then WiFi or if you wanna  download music or videos or images it will take some time.