I have both a 15" rMBP (2012) and a 12" rMB (2015 M5/512GB) and I love my 12". I use my 12" to browsing, music, look at video and more and I love the small factor of the computer.
Today I only use the 15" for video editing (FCPX) and will replace both of them when Apple releases the next version of the 12" as it is powerful enough for my edits (1080), even if the rendering takes longer time. For everything else it is powerful enough and the portability of it is so fantastic.
I have had a MB Air 13" and I loved it until I replaced it with my 15". It is a big difference on the non-retina and the retina screens, and I will never buy a computer without a retina screen even if the Air is a good computer.
I use Apple Music Match for my iTunes library and a Plex Cloud (Google Drive) as my video/media server. I love to have everything in the cloud to be able to stream/download it to my Apple TV / iPhone / iPad / computers.
Hmmmm... I'll have to go down and have another look at them side by side. The more I look at the Airs screen the more I hate the bad resolution and awful bezzle edge.
So how does the Plex cloud work? Would I be able to put my 180GB there? Do I pay for that amount? And will I have to download it to listen?
Also, will the 8GB memory make any difference to an upgraded 16GB?