I'd say stick to the iPhoto Library if that's the app you're going to continue using for now.
When you eventually swap to Photos, do the import again.
The Photos Library is greyed out if you attempt to open it from iPhoto. Any changes you make to the iPhoto Library will not be reflected in the Photos Library, meaning you'll gradually go out of sync until such time as iPhoto no longer works (OS X 10.11, who knows?) & you'll need to reimport at that time.
Photos Library Storage Space
The Photos Library is actually just a collection of hard links to the actual data on the disk & therefore is not taking up anything like the amount of space that Finder is reporting.
More on hard links from Ars Technica:
A hard link is simply a reference to some data on disk. Think of a file as a combination of a name and a pointer to some data. Deleting a file really means deleting the name portion of that duo. When there are no more names pointing to a particular piece of data disk, then that disk space can be reused."
This means that so long as one of the pointers to the data still exists, the data is still in use. Only by deleting both will the data actually be deleted.
Ref: Apple KB - Photos saves disk space by sharing images with your iPhoto or Aperture libraries