Which is faster--transfer phone to phone, or iCloud transfer?

Note also that phone to phone transfer transfers several configuration parameters that are not restored by a restore from an iCloud backup, making it faster because you don't need to reenter that information yourself.

https://www.howtogeek.com/191546/is-bluetooth-faster-than-wi-fi/

The Bluetooth standard’s maximum data rate is, at best, 3 Mbps or about 2.1 after protocol overheads take their cut.

Wi-Fi on the other hand, maxes out anywhere from 54 to 1300+ Mbps depending on whether you have “g”, “n”, or “ac” Wi-Fi plus any enhancements in the adaptors and router support.

Bluetooth is not faster than Wi-Fi. Not even vaguely close.

Bluetooth version 3 and version 4 have higher data rates, but the actual data transfer in those implementations happens over Wi-Fi; Bluetooth is only used to set up and negotiate the connection.

In my experience, the "fastest" transfer is from a backup of the iPhone stored on a Mac using a wired connection to the iPhone. The OP did not ask about this option though.

common sense….

imagine you have a bottle of wine in the kitchen, and you want to bring it to the living room

what is faster… bring it there directly, or bring it first to the bedroom and then to the living room?

Does that answer it?

If you use quick start the transfer only transfers iOS settings (including the app catalog and app data), which only takes a couple of minutes. It then switches to iCloud to complete the transfer, which includes downloading all 3rd party installed apps from the app store. Finally, if you have non-iTunes store music (ripped CDs or MP3s) that can only be synced from your computer, unless you have Apple Music.

Concur with this last.

And besides, phone-to-phone uses Wi-Fi, too; it just discovers using Bluetooth. It's pretty neat, and especially if you've got someone not using iTunes/Finder for any other kind of sync (PC-free) but USB, even at the lazy pace of 480 MB/s, still manages to be faster with a two-hop transfer from phone to PC to phone again.

Note too that what's at issue in the transfer is user data, whatever the method; apps always come from the cloud, and that's usually where a lot of the heft is.