Cellular preferred means it will only use wifi calling when you're out of coverage but on a wifi network. Wifi preferred means it will always try to initiate calls if you're on a wifi network through the wifi network, falling back to cellular if its too slow or has no outbound route. Neither is "better" per say, all depends on your preference. I tend to stick with wifi preferred, only because unlike project fi, tmobiles wifi calling will not transition from cellular to wifi, only the other way around. As a result, if i walk out of my building at work, while im still in coverage, I transition to the nearest tower, but when i walk back in, the signal can't penetrate and drops the call. With wifi preferred, I stay connected via wifi outside the building (unless i roam too far) and it allows me to continue my call walking in and out as i so choose.