What's keeping you in Japan? Is there something specifically that you don't think you could find in the US? How do you feel about the capitalism(healthcare etc...) system in the US?
Personally I feel like the US one could be good but it really depends on how much you enjoy Japan.
Just general differences in lifestyle and culture. I've gotten used to living here. There are places I like to go and foods I like to eat and friends that I have made. Saving money on the other hand is important so I feel pretty torn.
Well, what about your personality? If what you just said it's the only thing keeping you back for me it would certainly be US. You will find new places to go and you will make new friends while being exposed to new experiences. I have lived in 4 countries now and in each I made friends, found a favorite restaurant and activity.
Personally I feel like the US one could be good but it really depends on how much you enjoy Japan.