If we are discussing the viability and sustainability of any particular thing we should be using the current industry standard numbers and a reasonable estimate of where the industry is going. There's no sense discussing whether option A is sustainable or not if nobody actually produces option A is outdated and the next decade of installations will use option B.
I can see value in both approaches. One is a test of what happens if you go right now and buy a PV module from a nearby store. This has the implication that the module has been in the store for a while and the raw material was mined and collected many years ago. Then you compare it to getting the power from the wall socket, which similar implications that the power plant that generates the power was built many years ago using old technology.
The other approach compare the state of the art PV module vs the state of the art power plant, both being built using the latest mining and construction methods.
The answer of what is more sustainable is going to depend a lot on context. Are we comparing what a person can do right now, or what we should be doing in the future? For powering websites the general answer from the industry is currently to build data centers in depopulated areas in countries with cold climates, near a hydro power plant and with plenty access to cooling water. This is however not a solution that you can apply if the requirement is that the server must be located in Spain.
