It's my understanding that the cable method will work even if there's not enough free space on the iPad to do wifi. But I've never used anything but wifi. Both should have the same result.
Agree w/ Lanny - I've used both methods - always do a backup before updating, either OTA or wired to iTunes (usually do both) - for minor updates, OTA is my preference if a good Wi-Fi connection is available; for a major update (i.e. number increase to the left of the decimal point as going from 11.x to 12.x), I'll usually go wired to iTunes, arguably a more reliable connection - some more information HERE (a little dated but still applicable) - BOTTOM LINE - either way has worked fine for me. Dave