wifi is faster. Not everyone has unlimited data. Those on say a 500mb plan can surf more often without it cutting into their plan. Also some places may not have the best signal. Wifi will give you that.
and then there is the whole uma thing but only certain carriers support that.
wifi is fantastic for retrieving large amounts of data with your blackberry without having to wait for the EDGE/edge network to render it. the 3G network is comparable to some popular broadband internet service providers, and therefore is almost an equivalent. then again, wifi gives you the option of using your phone to transfer data versus using your actual wireless network (cell phone -> provider) connection to transfer data. i use wifi whenever i'm at home (even though i've not compared the transfer speeds using 3G versus using wifi at my house.
