I have three
Amazon.com: Godox TT560 Flash Speedlite for Canon Nikon Pentax Olympus + White Flash Diffuser: Camera & Photo and one SB700 but I've never used the Godox on camera. So I can't say anything really about them, outside of you get what you pay for. And knew that when I bought them but bought them with the intention of using them off camera.
I use a Nikon flash (SB700) for a primary (on camera) flash.
The SB700 to put it simple is a point and shoot flash in the TTL mode (through the lens) it will make all the settings for you. Or if you like you can switch into manual mode and make all your own setting.
The most common complaint I have read on the SB700, some claim it is underpowered compared to other Nikon brand flashes. If you look at the numbers it does have a weaker output, but in photo's I think you can't tell the difference and I think some people are trying to pick pepper out of fly poop just because they know it has a weaker output.
My advice is buy the SB700 and stay away from the SB600. Learn how to use a flash without blasting your subject with it (use the bounce card or bounce the flash off something) and you will never look back.
The SB600 had its day, but the SB700 is the replacement to it. I know someone is going to say something about the SB400, so I 'm going to save some time and give you my quick thought on that.
I may stand corrected here but it is my understanding the SB400 is no better than your pop up flash. The only difference is the SB400 "lens", not the whole head, tilts up and down and does not swing left or right. I don't know what you're looking to do with your flash photography but I would stay away from it if it were me.
I have a question to the other posters that brought the point of circuit damage, does this hold true to radio triggers? Or does this only hold true to 3rd party flashes when they fire might back feed for a lack of a better word into the camera. I'm using cheap third party radio triggers and the thought came to mind now.