In a word, either.
I use keel simply because I always have, like mode 1 on my radios.
Gary gives us comprehensive instructions about this item and in my experience of following such, and how my boats perform, stick to it!.
When talking about the keel you will invariably see mention the keel (or fin) box.
Make no mistake about this, the box and the keel MUST fit together properly and the box must then bit fitted to the hull as described.
It is best to cut out the keel first then make the box to fit around it, and this don\e before you fit the box to the hull.
When complete, the keel/bulb unit must (go all the way to the box cover and stop there, the supporting bolt holes need to be sitting there , waiting for you to place the locking bolts without a lot of fiddling about. It is therefore sensible to do this while the box is not permanently glued into the hull.
I just do it this way and then “tack” the box into the hull lightly while I check the alignment of everything to the hull.
That is another post, yet to be added..