The signal to noise ratio is 104 decibels in case of U5 and with the U7 it's 114 db. So U7 is higher in the audiophile food chain with more lossless transmission of signal. However for general gaming and Home theater experiences U5 should suffice.
Also, the U7 comes with Dolby® Home Theater v4 support. Can't tell you how exactly better the experience is due to this feature.