most TV's arent truly 120hz. Go with the monitor, 120Hz tv's are almost always post-processing trickery. I think there's only 2 true 120hz tv's and they advertise 480hz
The only tv you can get 120hz on will be a 120hz panel 4k tv in 1080p resolution because HDMI doesn't support over 60hz at 4k yet. So the monitor will be better as a whole with better input lag and response time.