[Q] Which Model topic

No doubt this is a question that has already been asked and answered. However, since I can't find it, I'm forced to bring this up one more time. I'm getting ready to shop for a new smartphone, and I am trying to choose between the S4 and the S5. It is my intent to get an unlocked phone, and I use primarily AT&T's networks. There are multiple variants of each of these phones out there, some of which will work for 4g LTE in the US on AT&T's networks, and some which will not. So, my first question is which models, or variants, of each of these devices do I want to consider? My second question is one of is it worth it to pay the premium for the S5 over the S4. I would have thought my timing was such that prices on the S4 would be dropping around now, just like they did on the S3 when the S4 came out. But I'm not seeing much in the way of a reduction in prices. If it's less than 0 premium for the S5, I guess I might as well bite the bullet. So, any direction or help will be greatly appreciated.