I have used both and they are both perfectly fine guide cameras. i would be happy with either
I have heard earlier ASI120 cameras have hardware issues related to USB, i can't verify that, but i do believe it's been addressed in the USB 3.0 versions
QHY camera is very sensitive and compact. QHY had issues with drivers for quite some time, but i've found they recently (within the last year) addressed that and it works fine now
I don't really have a preference one way or the other, so maybe it boils down to price? i would definitely not buy a used 120