i lost the thread where this started, but anyway..
i tested all my snes consoles (pal, us and sfc) next to each other, all with 75 ohm cables (which i was told were pal only) and they all looked the same.. i then tested with a cable i removed the resistor from, the picture was only a tiny fraction brighter, almost unnoticable.. i had to sit in fron tof the tv, swapping the video cables back and forth a few times before i could even be sure..
They're not the same. They really don't have that resistor inside them in NTSC land. And they are interchangable if you don't mind the video being less than perfect. Also remember that some newer TVs can make a beautiful picture from really shitty video (and for some it's vice versa :) ).
Here's my little table again. With and without the resistor. The number is a percentage of normal amplitude.
. With Without
.SNES PAL 100 130
.N64 PAL 95 185
.SNES NTSC 60 100
Whether really matters or not is debatable. I remember there was fair differece between a 'NTSC SNES with' and 'PAL SNES without' but definately watchable. 'PAL N64 without' was horrible though. This was on a ~15 year old Sony TV.