WiFi and 3G are compared in this article from The Feature:
Wi-Fi enjoys a number of advantages over the incumbent 3G. First of all, its being deployed, to varying degrees, throughout the world. Secondly, its cheap. Compared to the extraordinary costs of building out a 3G network – $10 billion for spectrum; $500,000 per base station; marketing costs; etc. – Wi-Fi is a drop in the bucket.
And finally, Wi-Fi is a grass roots movement that isnt suffering under the weight of bloated expectations and interminable delays. Its the loveable underdog. But dont confuse this fascinating technological duel with a zero sum game. This is not winner takes all. Rather, both these technologies will grow to rely on each other in time. And the result will be what weve all been waiting too long for: seamless wireless access at blazing speeds.
