USB would be better on battery life as the phone would charge whilst connected and not deplete your battery quite so much... as for speeds, this is down to the signal from your phone to the network and not your phone to your laptop/pc.
Unless you're in an area with lots of other Wireless networks or sitting next to your microwave whilst using it (both of which can cause interference) then the Wifi tethering will be just as good as the USB since your download speed is only 6-7Mbps at most.
Putting your phone on the window or somewhere it gets a good network signal may help your speed.



