"The Google Play edition phones automatically receive updates of the latest Android software. Optimized for the latest apps, more storage for your content and a fast, clean user experience all come standard. With an unlocked smartphone from Google Play, you can find the service plan that suits your needs. Upgrade your handset with no carrier commitment or contract. Unlocked means world travel is easy. Pick up prepaid plans as needed, or get a month-to-month contract with the carrier of your choice," explains Google.
The Verge tested both phones:
HTC's camera in particular managed to get slightly sharper shots in extremely low-light settings with Sense than the stock version. On both the GS4 and the One I found that video was slightly better on the skinned versions as well, with richer colors. (...)
The stock versions of both the Galaxy S4 and the HTC One well outperformed the skinned versions. In the Verge Battery Test (our standard test that cycles through a series of popular websites and high-res images with brightness set to 65 percent) each phone came in at about six hours. In HTC's case, that's a full hour longer than the Sense version managed to pull off. (...)
Technically, the "stock" Android on these phones doesn't come directly from Google (as with the Nexus line), but instead is built and maintained by Samsung and HTC. Google says that both phones will receive timely updates, but there could be an added wait from Samsung or HTC when the next version comes out.
An interesting tidbit: Google Play has over 975,000 apps and games.