Why not just get a T-01C? They don't go for much more than the IS04 (18k for the T-01C, 13k for the IS04 last time I checked, should be less by the time you go) and it'll work as a normal smartphone in both Japan and China.
If you plan on getting a MiFi like you did last time, and you're flying into Narita, know that 'Air's' has a great combo deal. 1100 (or was it 1050?) yen per day, including (up to) 2 free rental docomo SIM cards. All you need to pay for is airtime. Deduct 50 yen for every day if you rent for 14+ days.
Only downside I can see is that the docomo sim cards don't have data/i-mode activated so you can't download things like i-appli (no osaifu-keitai). But, with a smartphone, just connect your phone to the MiFi and download the apps over WiFi (don't think you can download i-appli over WiFi on 'normal' keitais?)
iPhone 5 [64GB|Black] | HTC One X LTE | NTT docomo N-06C
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