Quality is not what makes these cables different. Cat5 is for 10BaseT and 100BaseT networks (up to 100Mbps) and Cat 6 is for 1000BaseT gigabit networks (~1000mbps)....
Most people do not need a gigabit network. For offices that deal with a lot of data flow, it does make a difference. But, since you most likely do not have a gigabit router... I would save the money and go with Cat5. However, for the long term, Cat6 is not a bad choice.