Pay extra and get the S5, or save money and get the S4. The S5 is flashier of course, but really isn't worth the extra cost, imo. I upgraded from the S3 to the S4 and in hindsight I shouldn't of done so. It was not anywhere near worth it as an upgrade and my S3 didn't and still doesn't have a locked bootloader. Most likely in getting the S4 you will be on mf3 or mk2, but being AT&T the S5 will have a locked bootloader as well. Contrary to popular belief this doesn't make it the end of the world for the S4 or S5 as they're still awesome phones. This boils down to what you want to do with your phone and if you're wanting to root and install custom roms with anything asides of the stock kernel. So back to the beginning. Get the S4 if saving money is important, get the S5 if latest and greatest is most important. Both are great choices and you won't go wrong with either unless of course you do want to install non touchwiz custom roms.
[ROM][2.3.6] AT&T Galaxy S II - CoZ ROM
[GUIDE] Prep for App2SD and why do a "Wipe data / factory reset"
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Samsung Galaxy S IV
Samsung Galaxy S III
Samsung Galaxy S II
Galaxy Note 10.1 GT-N8013