NITPICK=ON
The 8500 and the 9500 logic boards are fairly different, mainly because the 9500 has six PCI slots and the 8500 has only 3. Might you have been thinking of the 9500 vs. the 9600? Or 8500 vs. 8600?
That would make more sense, as I've also heard that the 9500 case is as much of a bear as the 8500 case, but that the 9600 case (and 8600 case) was nice.
NITPICK=OFF
To sum up. The 8500 case sucks. I've heard that the 9500 case sucks too. The 9600 case and 8600 case were quite nice with the fold out panel for the logic board. The 9500 and 9600 logic boards have the same form factor. The 8500 and 8600 logic boards have the same form factor. However, the x600 versions used a different power supply connector than the x500 versions. Other than the power supply connector, the only difference is that the ROM was updated on the x600. The x500 has the $77D.28F1 ROM. The x600 has the $77d.34F2 (IIRC) ROM. The ROM chips are labeled 341S0168 through 341S0171 and 341S0280 through 341S0283 respectively. This only applies to the 8600 and 9600. The 7600 still had the original x500 ROM. The 7300 got the 0280 - 0283 ROM. I can't remember whether the power supply connector switch happened on the 7600 or the 7300.
Just for completeness sake...when the Kansas/MachV machines came out, Apple kept the nice 8600 and 9600 case and kept the x600 power supply pinout, but changed the motherboard again. The primary change was some kind of power supply pin change in the CPU slot (3.3V supply?) and the ROM, again. The Kansas (AKA 8600 Enhanced and 9600 Enhanced) has $77D.34F5, 341S0380 through 341S0383 for the ROM.
Incidentally, if you install the Kansas ROM in one of the older machines, then Speculative Processing on G3 processors works without any work arounds.