From a hardware POV, PC was more similar to S100 than it was to Apple II. The S100 bus was a dumb extension of the 8080 pins, the PC a dumb extension of the 8088 pins and the Apple II a dumb extension of the 6502 pins. The 6502 and 808x micros had very different timing cycles.
In comparison the fact that PC & Apple II used a motherboard architecture while S100 used a backplane architecture is very superficial. It was possible to buy S100 systems that used a motherboard architecture, it's essentially just hardwiring a connector. Here's one: http://www.s100computers.com/Hardware%20Folder/HeathZenith/M...
The other comparison point was in ecosystem: the S100 had a very rich ecosystem of vendors putting out systems and expansion cards, just like the PC Clone ecosystem.
Finally, the S100 and PC were open systems, whereas Apple was quite closed.
In comparison the fact that PC & Apple II used a motherboard architecture while S100 used a backplane architecture is very superficial. It was possible to buy S100 systems that used a motherboard architecture, it's essentially just hardwiring a connector. Here's one: http://www.s100computers.com/Hardware%20Folder/HeathZenith/M...
The other comparison point was in ecosystem: the S100 had a very rich ecosystem of vendors putting out systems and expansion cards, just like the PC Clone ecosystem.
Finally, the S100 and PC were open systems, whereas Apple was quite closed.
In 4 different ways I see PC as S100 v2.