Use both. Having the cryptic helps filter out the water. The macro makes a good neutrient exporter and at night, when the PH typically drops, having the lights on in the fuge will keep it stable. Especially if you have macros growing in your main display - if not, keep the refuge lights on 24/7 - don't use Caulerpa.
A DSB with crptos and some chaeto will make for a really good refugium.