BTW - have you decided on what to do after Aperture yet? I can't find a suitable replacement yet and will just hang on to Aperture for now. Photos is far too limiting at this point for me.
I keep my archive in Aperture for the time being and a smaller library with the photos I am currently working on in Photos. I am lucky, because the basic structure of the Photos Library with Moments - Collections - Years is exactly the structure I used too for my Aperture projects anyway. Now I am getting it for free. In addition to that I used keywords and smart albums, and the keywords migrated nearly lossleesly from Aperture.
The main reason I am trying out Photos is, that I like iCloud Photo Library to keep the photo library in sync across all my Macs. I can start editing on the iMac and continue on one of my my MacBook Pros.
I started to migrate from Aperture to digiKam, because it is very versatile and supports all I would ever need in a photo application. And it would make it easy to switch from a Mac to back to Linux, but I cannot recommend it at the moment. With each system upgrade I would have to wait for an compatibility update and start over again with reinstalling the MacPorts and X. It would become tedious in the long run. It worked very well in Yosemite, but I could not find an El Capitan compatible version yet. Not being able to upgrade the system because the photo application is not supported on the new system would be too restrictive.
If Apple keeps improving Photos and more powerful photo editing extensions become available, I will stick with Photos.