Welcome to the forums and hopefully soon, Android!!
I do not have the S4, but the S3. Out of the box, you can highly customize your phone, from wallpapers, ringtones and notification tones, to widgets and launchers. Launchers are a different User Interface that by themselves can offer more changed to the device. You can change scrolling animations and speed, color themes, grid layout (number of App rows and columns), the list goes on and on with just launchers. All of this can be done without root access (Jailbreaking). Once root access comes into play, you can change ROMs, and that offers a whole new selection in customizing.
Best thing to do, and this is the fun part....go into AT&T, and play with phones. I would take several trips into stores just playing. I think AT&T already has the Moto X, the LG G2 is due out soon, as well as the Galaxy Note 3. Now, Samsung does have a history of subpar cell radios, but that doesn't mean they suck. They could just be better.
Something else to do before you go into stores: look up features of the phones. Literally make a list of features. As you figure out which ones you like circle them, and cross out ones you don't like.
The HTC One is a hit with longtime iPhone users. I will agree it is an extremely good phone. I love the screen, wish I could have it on my S3. However, one thing I like about Samsungs is the external SD card. I put music, pics, and ROM backups on that leaving more room for apps. Be advised: The 16 GB S4 will only have about 8.5 GB of user space after formatting, Android use and Samsung features. So, if the S4 is something you want, try to get the 32 GB.
I know I have thrown a lot at you. Any questions, ask away...