If you want updates as quickly as possible it will be best to get it from the play store.  They do not sell CDMA versions as they work differently so updates will come but not as fast as GSM versions.

ann beat me to it lol

a side.  No carrier will ever give a date.  Especially since the MMS lawsuits that happened when AT&T said MMS was coming to the iPhone.  They said end of summer and the day after the end of summer 8 lawsuits were filed.  So I don't expect any carrier or manufacturer ever give an end date, but will give a beginning date (press releases stating these devices will be updated etc.)