Emulsion seems to be the trend among some of the top manufacturers, but not all.

As mentioned by others, emulsion adds air to the oil, to allow for the volume compensation that the shock shaft displaces when pushed in. Also suggested is that the added air slightly dilutes the oil, reducing its effective viscosity.

It is possible to build zero rebound or "dead" shocks with either build method. Whether one is better than the other on a specific track, for a specific driver simply needs to be tested.

The easiest thing to do is to find another set of shocks. Build them identically, aside from bladder or emulsion. Or, just have the O-rings, bladders and/or shock caps available to easily switch between the build types on a single set of shocks.

Run some laps with each. Swap them out. Play with the oil weights and spring choices. As much as I love racing, a good test and tune day is quite rewarding. Remove the guess work. Become your own "pro" at tuning and making educated changes to your car on race day.