Have used ervocom and touch lines and signal bip as an assistant. Never used buzzers as AR.
So I'll stick to my experiences actually using the flag
For me the build quality of the touchline appears better. Lost count the amount of ervos I have been given where the buttons are breaking/broken.
The feel of the touch lines are better as well, as it's a flag I am used to using (non buzzer version). Also the length is better, I always find the ervocom flags can get trodden on as I run down the line as the stick and flag is a little longer and I am not the tallest.
Price, in the UK the touchline come out on top here too.
The signal bios don't feel as quality but I have had no issues using it. I don't. Think you can turn the beeps off on these though which I think I'd want to in a buzzer flag.
There may be some big differences in the refs experience which I can't comment on but certainly as an AR I am much happier if the ref hands me a touchline flag