If you aren't familiar with the manual installation process, or if you hate running into problems, you should probably skip the manual upgrade in favor of the OTA.
While the manual update will get the software on board your device early, it also has the potential to harm your device. Veteran Android users shouldn't have any problems but if you're new to Android or you're not confident in your abilities, you'll want to wait for the OTA to arrive. It's as simple as that.
We'll take it a step further too. You might not want to install the OTA the second it arrives either. Android updates, especially big system updates like Android 6.0, have the potential to bring bugs and negatively impact performance.
If you wait a few days, you'll be able to survey the landscape and get a better read on the update's performance. Remember, Google won't make you download this update so you'll want to take your time.