Always with iTunes, which will download the full iOS file. And the OTA delta update might be a smaller file, but it won't work if your device is tight on storage space (it needs a LOT more space than just the size of the update file). The iTunes update will work regardless of how much space you have on the device itself, because the files are unpacked on the computer.
Part of my maintenance routine involves doing a factory reset and restore from backup about once every other month. I need to do this because the "Other" category (I suspect these are mostly failed iTunes Match downloads) and some of my apps will accumulate a huge amount of storage space and won't release it until I reset and restore. Just the other day, I cleared more than 3 GB when doing the reset and restore.
Having the full iOS file on my computer allows for the factory reset and restore from backup to move much faster. Also, I just like the idea of doing a clean installation when updating the OS.
