First off, thanks for putting in the work to get this thread (and art) started.
I think you've presented the question as somewhat of a false dichotomy. Obviously the icons are not going to be ~300x300px pencil sketches. You need more sharpness and clarity for functional icons. You know that, but still...
I think the important thing to consider is weather they should be
1) symbolic and iconic vs.
2) natural and "photographic."
Both your examples are very #2.
I think Geoff's interest is probably that the building icons are stylistically distinct from the tech icons to help make everything easy to sort and identify. Weather or not this is a worth goal, but there's more to consider.
* We don't have a lot of room. point for #1
* Many or most of these building types don't have intuitive structural distinctive. A person could easily guess what a "planetary ring" looks like, but not a "hyperspatial dam" or "collective thought network", or "theorem prover" etc. Recognizable images for the less obvious class of buildings would naturally tend towards iconic.
* The more we style the building icons art to look like an actual view of the buildings, the more obvious it will be that this building is not appropriate to each alien species and/or each planet type. Architecture would obviously going to be very different on a lava, terran, and ocean worlds, let alone as built by different beings. The more the more each icon looks like a picture of a building built by a particular species on a particular planet type, the sense of "wrongness". And don't even suggest that we make alternate icons for each planet type and/or species. That would make the UI absurd.
So in summation, i think we should use an SMAC-like icon style for the buildings, probably ultimately color-coded like the tech icons to reflect the category. Making them distinct from the tech icons in some way would be good, but that's a secondary concern. I don't have any ideas on how to do that at the moment.