There are two big pluses to the Spanish heel method:
1. Getting the neck alignment and angle is easy. You glue the soundboard to the neck (having chiselled a step in the end of the neck to accommodate it) and then clamp the lot face down to a board. Neck angle is adjusted by placing a shim under the nut/zero fret position. If you want a domed soundboard you dish your workboard or build up its edges.
2. You don't need a mould if you're not fussed about perfect symmetry in the body shape. I just glue blocks on the soundboard around the perimeter of the body, and then build up from there, using the blocks to ensure the sides are aligned with the body shape. As I rarely build two the same shape, this saves me a lot of effort.
For me there's a possible 3, as I get the neatest neck/body seam using the Spanish heel.
Finishing and binding are much harder though, and that's the downside.
I'd say Spanish heel is the easiest for a first build, but that's probably because it suits the way I work best. Second easiest is a bolt-on.