Loading a bottom-loading camera means opening the bottom, putting it somewhere, pulling out the film leader, dropping the end of the film leader into the spool, dropping the canister into the other slot, closing the bottom again, cocking the shutter a bit while watching the rewind, and advancing the film a frame or two.
Loading a back-loading camera basically means opening the back, dropping the end of the film leader in the spool, cocking the shutter a bit while watching the spool, pulling out the film leader by the canister, dropping the canister into the slot, closing the back again, and advancing the film a frame or two.
So where is the big advantage of either?