I have 2 choronographs, a Pact and the CED M2
. Unfortunately, I don't have experience with the Prochrono.
My Pact gives consistently higher readings. If you are only interested in 'relative' readings and are more interested in your velocity spreads, most Chrono's will work for you (just be consistent with your bullet placement over the sensors, same height/centered). If you want to know the actual velocity, I'd recommend the CED M2.
Your mileage may vary, but the velocities I get from the CED match my estimates from Quickload. The drop tables from Exbal using the CED velocities have always matched the drops I see in the field. So, in my experience, the CED is right on and the money. My Pact is 5% or so over.
Take some measurements with your Chrono and see if they match your 'expected' velocities and your field data (from a drops perspective).
Hope this helps,