My tests indicate that wifi consumes a significantly higher amount of battery life. Last night I browsed to 10 different websites, first on 3G, then let my phone sit for the remainder of an hour.
I then repeated this(and should have been easier on data considering the sites are now cached.) and let it sit for an hour. After each hour I examined the battery use application in settings.
The first time, under 3G, the display topped it at 46% with everything else under 9% each. The second time, Display was down to 37% and and WiFi was now listed as 26%.
Cell standby was 3% and phone idle was 4%.
let me know if you have any questions.
I intend to re run this test again tonight and will try to post screen shots if someone can recommend a good screen shot app.