Both Firewire and USB 3.0 were developed with ease of use in mind, and as a result neither is particularly difficult. They're both plug-and-play and won't typically require the installation of drivers in order to function correctly. It is unlikely that anyone will find one harder to use than another. Besides performance, there are not any features which are substantially different for the average user.
At this point USB 3.0 is looking good. It performs the best and doesn't have any hidden disadvantages. Unfortunately, there is a catch. USB 3.0 is new, and its adoption seems to be taking some time. While there is a small number of devices currently on the market which support USB 3.0, the popularity of USB 3.0 doesn't seem to be growing as quickly as one might expect.
This puts a real limit on the excitement surrounding USB 3.0. Firewire may be slower, but it isn't terrible, and it is much easier to find Firewire devices than USB 3.0 devices at this time.