The main additional feature on the PRO 24 DSP (from the PRO 24) is an onboard DSP chip which powers the Virtual Reference Monitoring feature, and tracking tools such as compression, limiting and comfort reverb without any strain on your computer's CPU.
It also comes with an extra headphone output on the front of the unit so two people can reference the mix using VRM. The PRO 24 DSP also has Dim and Mute switches on the front panel.