As a generalisation, Docomo have best coverage then AU then Vodafone.
I went with AU because they had a phone that I liked. Had a good camera, and was a little smaller than some others. Prices are pretty reasonable, obviously depends on how much you use it.
Also I don't know about other carriers but u can use C-mail with AU. Its like SMS, cheaper and smaller than an email, but can only send to other AU people.
Also check things like english to japanese dictionary on phone. As well as English dictionary with predictive text. My phone does not have predictive text, so it takes a little longer to type mail. But for the translator you can use an online one, Tango Town is what I use.
BTW my phone is a W41CA.