I keep both chaeto and grape caulerpa in my refugium. I haven't had a positive reading of nitrates for approx. 5 months now and both are still growing.
I wil point out that if you have elevated levels of phosphates then the caulerpa will be your best friend. If you run a super low phosphate level its better to go with chaeto.
Keeping both of the algaes in my refugium I was at first getting a little over a pound a month of caulerpa growing in my 30g system while at this point my chaeto wasn't growing at all. Since I added phosphate removal media the growth pattern has reversed and the chaeto has started growing out of control.
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor