“Wifi is on average three-and-a-half times more dominant than 3G when it comes to delivering mobile internet data services,” said David Gosen, managing director for digital at Nielsen Europe. “It peaks around midnight as users gravitate towards social networks, driven by their desire to stay connected through all waking hours."The big difference is pricing, I suspect.
Source - Wifi carries 78% of UK Android data, telecoms.com
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