Just a quick note about the new Photos app in 10.10.3; there’s currently no way to disable the auto-generation of previews or thumbnails, so be prepared to lose a lot of disk space for your new library.
For a quick example, I have ~85GB of photos (around 9200 files) that I store on a secondary 2TB HDD, since my OS and applications are on a (relatively) small, 250GB SDD. I do however keep the various library files (e.g. all the metadata) on my SSD so the apps still startup very quickly. My old Aperture library, was around 1GB for 9200 photos, because I disabled automatic preview generation. My lightroom 5 library for the same 9200 photos is very similar, around 900MB.
The same library of 9200 photos converted to the new Photos app? 12.18GB