Google announced the Google Edition Samsung Galaxy S4 at Google I/O in May. HTC later confirmed that it, too, would also release a Google Edition HTC One, arriving on the same date: June 26.
According to the Google Play listings, both devices will begin shipping by July 9. Google is also offering free shipping.
In addition to being stock Android devices, both Google Editions (some are calling them "Nexified" or "Nexus Editions") come unlocked and will work on any compatible GSM carrier, including T-Mobile and AT&T. Both come with LTE support, and the device both have an unlocked bootloader, which will make rooting and custom ROMs easier.
The stock Android version means that both will receive ROM updates faster than standard devices. Both ship with Android 4.2.2, the latest as of the time of this writing. The reason we say they are "more or less" stock Android is because there is software on the phones that does not come from Google.
For example, the phones run kernels supplied by their respective OEMs. That, however, shouldn't be surprising. As non-true Nexus devices without the typical Nexus development cycle, the OEMs had to step in and "assist" in "Nexifying" them.
Unlocked and sans carrier subsidy, they are both pricey. The HTC One is $599, and the Galaxy S4 is $649.
The GS4 ships with 16GB of internal storage, and the HTC One with 32GB of internal storage. However, the GS4 supports microSD, which the HTC One does not.
It's still unknown how much internal storage is "available" to end users. When the Galaxy S4 launched, Samsung was hammered over the amount of internal storage that was left after all of Samsung's bloatware and TouchWiz were installed.
It's expected that a stock Android version will do much better in that regard.