Let's just nip this in the bud right now so more people won't get confused.
The region of the console itself doesn't really matter, as a) clockspeeds are largely the same and b) any N64 is capable of outputting both PAL or NTSC signals. Older consoles (NES, SNES, Genesis, Saturn) will have different base clocks to compensate for the different framerates, but consoles after a certain point (N64, PlayStation, Dreamcast) will allow the software being loaded to choose the video standard.
The output video standard is decided by the software (i.e. the game). A PAL game in
any N64 will output a PAL-encoded 50Hz signal while an NTSC game in
any N64 will produce an NTSC-encoded 60Hz signal.
In the case of the N64, if there are any differences in the speed a game runs it's
solely down to how the game was programmed. In any region the N64 hardware itself is similar enough not to be a factor when it comes to speed.
You typically cannot run an NTSC cartridge on a PAL N64 or a PAL cartridge on a NTSC N64 because of the CIC lockout chip. If the CIC in the cartridge doesn't match the CIC in the console, the cartrdige will not boot. If they were able to boot, the output and video frequency would still be exactly the same.
Flash carts (i.e.
64drive) can allow the loaded games to do run as they normally would, but they can also force a game to output the
other signal. UltraHDMI apparently has the same capability. This means that you
could force a PAL game to output a 60Hz or an NTSC game to output a 50Hz signal. Very important to note that doing this will
not affect the speed at which the game itself runs. Forcing a PAL N64 game to output at 60Hz won't make it faster, and forcing an NTSC N64 game to output 50Hz won't make it slower.
To summarize:
• The video standard to use is decided by the software.
• The only relevant difference between N64s of different regions is the lockout chip, nothing video related.
• If you have NTSC cartridges, get an NTSC N64.
• If you have PAL cartridges, get a PAL N64.
• If you have a flash cart, it doesn't really matter one way or the other as they can load/play/do everything.
If you live in a PAL country, the only point of importing a US N64 would be if you happen to have a ton of NTSC games lying around, but I don't think that's the case for most people.
A 64drive combined with UltraHDMI is the ultimate combo for any N64.
Hopefully no mistakes crept in there...