All very true, I was going to suggest ebay but I didn't realize they were so cheap on amazon, the Z431 is only $36.99 there.
Unfortunately I am a colossal data hog which is why AIO makes sense for me! I have an iphone 5 which makes no sense at this point but it is paid for and unlocked. I have my old iphone 3g as backup if the 5 ever dies. AIO's big draw for me is even their cheap plan offers unlimited data. You just get 250MB at 'high speed' and then after that you are throttled to a low speed. But that low speed is more than enough for email, occasional surfing, maps and pandora so $40 for me works out really well. Especially since I get a $50 a month supplement from work for the cell phone since I am required to be in contact.