NEW: BETTER SOLUTION:
If set up optimally, is USB or WIFI faster when syncing an iPhone 4 with iOS 5 to iTunes?
Wi-Fi is MUCH FASTER.
Lifehacker and company are wrong IF things are set up well/optimally!!
Optimal: Use 2.4 GHz Wi-Fi ONLY between the iDevice and the router. Use a wired (Fast or Gigabit) Ethernet connection or 5 GHz between the router and PC/Mac.
USB 2 is 12 Mbit/s (max), but one can sync over Wi-Fi 802.11n, at much higher speeds, if you avoid heavy contention for the Wi-Fi bandwidth. I just ran some tests (6 test runs, plus warm-ups) and was seeing iTunes sync speeds close to USB 2's max over a cable, and 2-4 times that speed (26Mbps to 50 Mbps) when connecting via a wired (gigabit ethernet) to a router and then over Wi-Fi to an iPhone 4S (that is, one wired hop and one wireless hop). (When sending large files to the iPhone. Short distance, 2.4 GHz 802.11n.) I also found that the basic no-op sync when nothing had changed was slightly faster over a well- configured Wi-Fi connection than over USB 2 (apple brand cable; no hub).
WHY? Data sent purely over Wi-Fi via a Wi-Fi router between a PC/Mac and an iDevice will result in heavy contention, as the data has to make two wireless hops, and Wi-Fi routers generally don't buffer well in that situation That's why a VoIP phone call using WiFi and bluetooth simultaneously is so often choppy: The WiFi and bluetooth are often competing for the same 2.4 GHz spectrum at the same time. This is also why most folks are getting poor iTunes throughput with Wi-Fi.