Hi Bill - Jabber Guest is supported on iOS and Android, as well as Mac and Windows via web client.
The experience across all OSes is similar, so it's really about the best device for the external user. Of course the web client will be used on a laptop/desktop, so will have more real estate to display video. The web client prompts the user to download a plug-in before making a the first call, after that they just click a link and go! Of course, if your external users need to dial in when they are not near a laptop, then mobile is the way to go!
Hope this helps.
Mike