As far as I know, DarkPHYSICS was the original physics plugin developed for DBPro in about 2006. It's still powerful and has amazingly realistic physics, but it's quite out of date and has a few bugs... Users need to install an old driver from Nvidia to play a game which uses DarkPHYSICS too, which isn't great :/
DarkDYNAMIX is newer physics plugin (first started in 2012... I think). I've never tried it myself, but apparently it's awesome too! One huge advantage it has over DarkPHYSICS is that users don't need to install any extra drivers to play games with Dynamix.
If you're wondering which to get, I'd definitely go for Dark Dynamix