Hi Rocktom,
In essence, the 'U' series of processors are ultra low power processors. They do tend to perform slightly below the M series, however in most cases they draw less than half the power than their respective M series processor. The U series are often grouped with lower power RAM and such, leading to a greater battery life.
Given that the U version you've shown is a newer generation than the M version you've posted, I would say that both processors will perform very very similarly for your tasks.