Unlike the Google version, which will be sold on the Play Store, these versions both appear to have TouchWiz on them. Both an AT&T page and a Verizon page appeared on Samsung's U.S. site. The AT&T version has disappeared for some reason, but the Verizon one still exists at the time of this writing.
The phone will come only in Black Mist, at least to start. It will ship with 16GB of storage memory, and no other internal storage option is mentioned on the Samsung site. Also not mentioned, but expected as this is a developer phone, is an unlocked bootloader.
To be clear, the Google version of the GS4 will ship with stock Android when it launches in June. Neither the AT&T (when the page was still up) nor the Verizon phone lists a price or a date, but we'd expect them to be sold unsubsidized, as most developer editions are.
In other words, don't expect a nice Google-subsidized price, as with the Nexus 4. Instead, the AT&T and Verizon versions will probably cost the same as the Google Play model: $649.
Dates are not available for any of these, except for the Google version, which is supposed to ship on June 26.