While rereading these posts, I notice multiple views in that a pull-up might or might not be a diaper. That is where I see the difference - the indecision, but that indecision is more to do, in my opinion, with useage rather than the product.
A diaper/nappy is something that is put on another, usually by a career of someone who needs to wear it i.e. a parent putting a diaper/nappy on a baby.
The pull-up can be placed on another, but usually is put on by the wearer.
Both products are supposed to fulfill the same function, but experience here tells us that the pull-up is a facimile, or a weak copy at best. The missing interaction is, what I suppose is what the baby / adult baby is looking for. The end feelings of bulk between the legs exist with both products, but the self perception of one as a baby when wearing a pull-up is non-existant. The image is wrong - to me.
When I look at the progress to pull-up type diapers for babies, I also see the link between older and older children still needing diapers in school and later in life, and I ask, is this an engineered progression by companies to increase protection usage as a way to increase profits? .... and also the source of the confusion of what a diaper/nappy is and what a pull-up is - and maybe the reason for the OP question?
Functionaly, changing a babies soiled pull-up diaper adds complication to an already hands on process. With a baby lying down, the task is easier. Where the baby is can stand, there is the complication of lifting the baby to a standing position, holding the baby, and then pulling up the pull-up diaper. The baby might wet a little when the weight of internal organs presses its bladder, so not only do you have wee all over the place, one has to restart the cleanup. I don't know how most here would do it, but in that case, I prefer the tape on diapers. Pull-ups are best used during toilet training etc - as they avoid any mistake in the childs mind between a diaper and underwear.