right. they both r the same.
if u're ready, ofcourse u're prepared for something. so there is no difference.
Dad: "are you ready yet?"
Mom: "yeah. I'm all ready. Let's go"
I'm all prepared also can be used when u had to make arrangements for something. for example a party and ur friends would ask if u have arranged everything already. U may say "yeah. I'm all prepared for the party tonight"