Both versions and app sizes are the same. Why have they got two extensions then?
Only Google can say for sure, but it seems to me that it is likely there were two different extensions originally, and they've been able to merge the codebases. Perhaps one had been originally for Chromebooks and the other for Chrome on other OSes.
And, while they've been able to make them identical, some people will have one installed and some the other. Rather than forcing some people to go through the process of uninstalling one and installing the other, they just made it so they're both the same. (They have, in the past, done it the other way, and it generated a lot of complaints and, I'm sure, a lot of people who didn't install the newer one.)
They recently changed the Hangouts extension so that it acts more like the smartphone app. That's probably part of it.
Extensions can appear in the extension bar.
Apps can appear on the apps screen.