This post doesn't do anybody any good as att no longer offers unlimited data to new customers.
KD, do you have any lines on your account without unlimited data? If so, there is a loophole to use it and keep your unlimited. Costco, or any location that uses verizon eROES activation system can run an alternate upgrade to the line without unlimited data, then just change the new device back over to your line. Presto, unlimited kept. I do it on a regular basis.
Costco refuses to work with apple, so I'm not too sure about your ability to get an iphone and keep unlimited, but I can give you 100% assurance it's doable at Costco, given the salesperson knows the loophole.
As far as phones, there's a reason the S4 is near-unanimously reviewed as the best smartphone out there. I've had the iPhone 3GS, 4s, 5 and now 5s, as well as samsung galaxy s, s3 and now s4. I'll admit the cloud services offered by apple are great if you're fully in the apple system (MacBook, iPad, Apple TV) but they're still ultimately overpriced given the hardware you get. Apples intransigence on device pricing, compared to th ability to find heavily discounted devices... Also the propietary nature of apple hardware can be a pain, whereas all android hardware supports regular old micro USB.
Another phone I would consider is the LG G2. I used to hate LG, but this phone is outstanding. I got my dad one for his birthday. Spectacular screen, snapdragon 800 (currently hands down the best mobile processor on the market), battery life is superb and a standard 32gb of storage compared to iPhones 16.