Sorry, late reply here... but are you a Japan resident?
Webmoney - these are prepaid card that you can buy but I don't think you can add funds to them once used.
Rakuten Money - I think this is just a wallet service like Yahoo! Wallet, etc.
I don't have any direct experience with prepaid cards except those visa cards I get as gifts from points earned while using my normal Japanese charge cards. Those can be recharged with the instructions included.