I sort of do both - but mainly the wall.
When I am at work, and I need to charge my phone, I will take the USB cable with me to work and charge it from the computer. This usually happens when I come home on lunch and see that my battery is now down to 40% or so. I have a battery which isn't holding the charge as much as it would normally so, I am waiting out my contract expiration before I go invest anymore on this phone.
I do make it a point though to charge my phone at night on the wall regardless of how charged it is.