I can’t say I’ve ever considered the differences between charging my Nokia Lumia 800 via USB and via the AC adaptor. If I had thought about, I would probably have assumed they charge at just the same rate. But this morning when I was thinking about going on a bike ride, I realised my phone was out of juice, so wanted to charge it ASAP. A little research showed using the AC adaptor would be much quicker than plugging into USB.
But in case you don’t believe me (And to be honest not believing random people on the internet is probably a good thing), you can check it out for yourself. From the phone dialler, type ##643# and the diagnostics tool will be installed. Run this app and look at the battery status section whilst charging. I found the AC adaptor was providing about twice as much power as the USB connection.