Re: slap or silent in ukemi?
I like to always run thing by the "Concrete Test." Don't practice an ukemi that you cannot do when being thrown on concrete. I think that Systema has a more realistic approach toward receiving energy (ukemi) in that you should conform to matter that is harder than yours.
The slapping habit can be dangerous when you are only practicing on soft surfaces. Hard surfaces allow you to discover how to reacquaint yourself safely with the ground. Although Ushiro Sensei is a karateka, we train on hard wood floors and I have had no problem doing ukemi and even full-force, sacrifice throws on the ground with no bruising. I thank the influence of Systema in learning how to better my ukemi skills.
Good Luck finding out what works for you.