3g has the convenience of being able to connect to the internet almost everywhere as long as you have reception. But if the reception is poor, it will result to a slow internet connection. It is more expensive than wi-fi, and having unlimited mobile internet really helps, since if you go past your monthly limit, you get charged with a hefty amount for every extra megabyte you use.
Wi-fi on the other hand serves as your internet connection. It is usually faster than 3g because you can easily just go closer to the router to get better reception. The main drawback is that you need to stay close to the router to get continuous connection, and this hinders your movement. It is better to use wi-fi when browsing webpages with lots of heavy multimedia, like music and videos. It is also better to use when downloading large files.
