Thanks for the picture. It really helps. Yeah curious how the PS5 design will age... I have it horizontally stored in the TV cabinet hidden behind a tinted glass door so the only visible thing from the outside are the game pads when not stored! All of that is in the living room (not a gamer's cave) and hidding it is a no-brainer for me: I find the design of the PS5 way too aggressive for non gamers (family, friends over etc), even for me when not in a gaming mood.
In terms of choosing the right PS3 model, what I am after are PS3 game disc compatibility (and the very occasional Audio CDs). PS2 BC would just be a "nice to have" so I wouldn't mind missing out or having it as a bonus (just like audio CD) with a lesser quality than the original PS2 (which I hear you is a much better UX).
Back to my TV cabinet. I would have just enough height to pile up PS5 and the original PS3 horizontally. As you said, PS3 phat is bigger than PS5 so the logical thing would be to pile PS3 on top of PS5. But: the PS5 top surface (when laid horizontally) isn't flat and also gets hot.
I was curious if anyone had been (aesthetically / functionally) successful at "layering" PS3 + PS5. Maybe this could be a 3D printing mini project.
Unless (last resort measure, I am not that desperate so far), I dig up
one of those bed tray tables at the flea market or ebay, high enough to accommodate the PS5...

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