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.
Mebbid's Fishtopia: Various builds
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor