I have used a few variations of FIST gear suits, some full body sets for Law Enforcement Training as well a some miss-mosh sets just using a helmet & thigh pads and other heavy groin padding etc..and whatever else I have access to. As long as the suit offers competent protection and the participants train hard & safe, most likely you can get as realistic as possible while still maintaining safety for participants.
A good suit does not make the "Ironman" practice assailant completely invincible and Murphys Law can cause terrible happenstance, inflicting injury, so always be aware of what is going down as best you can. I do feel that the FIST gear is better from my experience, but it all depends on the people training.
I have seen some foolhardy LEO's with minimal training experience do silly things with the top of the line FIST gear suits and equipment out of a Gung-Ho attitude, while I have seen others with minimal gear fashioned from scrap & older athletic gear to produce some greatly effective realistic training from emptyhand sparring & self defense drills to full contact stick, dagger (FMAs) & mock firearms defense drills.
Common sense & experience should define acceptable equipment to be used for your desired level of training. I am sure others may chime in to give their input. Best of luck finding good gear, have fun and train safe. PEACE