The walk light will often be on at the same time as the green light for drivers going the same direction. Sometimes a turning driver won't notice you. Doesn't happen that often, but happens often enough that you have to be aware of it.

A lot of times it is just easier to cross in the middle of a block because you have a much better buffer area where random cars can't pop out from. Obviously it depends on the particular road. If it is some 6 lane wide main thoroughfare, I'd probably opt for the cross walk.