Actual Bluetooth is slow, 1.5Mbit or so, depending on the implementation. You wouldn't transfer large amounts of data that way. Maybe a photo, or smallish files. But, as the other poster noted, Airdrop uses Wi-Fi direct now so it's at wifi speeds.
Modern Wi-Fi 6 (what is in the new iPhones, even in the older iPhones) is theoretically much faster. Gigabit+ speeds. In practice, you can expect similar or faster speeds vs. USB2 over Wi-Fi. If it's ad-hoc from iPhone to iPhone, it should be much closer to theoretical max since they will be in close proximity.
