Smart Switch IS OTA, it's just using a different program to do it, so both aren't preferable. OTA just means that you get the update file over the air - whether you get it using the update program in the phone or the update program in Smart Switch makes no difference, if the download is bad [because some of the data downloaded didn't get to you properly], you're updating with a bad download.)
Neither voids the warranty, but if the update hangs, the phone has to be brought or sent in for repair. If the carrier does it with a non-OTA file, they've used the same file before, so it works, so it won't cause the problem.