Um...they're the same system.
Seriously. Same thing. The same code running on the same processor. They flipped a couple of pins on the HuCard slot and made everything sold in the US hella ugly. None of this makes them different systems.
If you want some US games, but them. If you want some JP games, buy them. I don't see much point in getting an actual TG16 system since it has unique controller ports and only works with one CDROM2 system.