Way too many variables involved. If you have a good USB cable that's a reputable brand the data transfer rate will be pretty high, plus a physical cable is impervious to the signal interference issues that are always present with WiFi, a wireless connection. But if you have a pretty new router, the WiFi signals it emits will likely to be using the more advanced, faster protocols so it could be faster than if you're using a cheap, knock-off brand USB cable.
Basically, your question is missing a lot of missing, relevant details. But for the most part, chose the option that works to be the most convenient for you. This particular task is essentially something you need to do only very occasionally. I'd pay attention more to making sure everything you're transferring over is intact and be sure to double-check that it does, worrying about the fastest way to do it shouldn't be a priority.