If you are mobile - 3G is usually better. It relies on GSM infrastructure so you can easily find network in each place you can use GSM phone. You have to pay for this service but it is not very expensive.
WiFi relies on "access points". Range of such access point is about 100m maximum and less than 20m inside buildings. There are no "unified" infrastructure for WiFi so you have to pay or have another type access permission in each place you try to use WiFi.
If you are thinking about browsing "everywhere" - 3G seems to be much better choice.
If you are thinking about home&garden or one company solution - WiFi can be more useful.
