Q: Should I keep the old iPhoto library?

Do a complete back up on a portable HD (preferably 1Tb).   Make it specifically for photographs.   Once it is securely backed up you can remove the iPhotos version.   Usually for sentimental reasons alone, a second back up is never too much either.  

Their rather large files, with iPhoto being just over 320BG and the Photos one just under 250GB. That's a lot of space on the hard drive.

Are you still occasionally using iPhoto?

If you Photos Library is a migrated iPhoto Library, the iPhoto Library and the Photos Library are sharing the original image files. The migration from iPhoto to Photos is just creating hard links to the image files in the iPhoto Library. This means, that the photos in Photos are stored on the same disk blocks as the photos in iPhoto.  The sizes you are seeing in the Finder are misleading. Both libraries are having a large size, but are stored mostly in the same place are are needing much less storage than the sum of the sizes. Deleting the iPhoto Libraries will not free much storages long as Photos is using the same images. 

See:  Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

So, if you are still using the iPhoto Library occasionally to look up old photo books or the descriptions of your events or similar, and are not running out of storage, keep the iPhoto library in place.

The problem is, that if you delete the library now and move it to an external drive, it will break the hard links. If you need to restore the iPhoto library to your system drive it will no longer be linked and you will need much more storage than now.

But if you do not use iPhoto any longer and have recovered all metadata from your albums, events, and finished all projects, do as seventy one recommended.  Photos does not need the iPhoto Library.

with iPhoto being just over 320BG and the Photos one just under 250GB.

There is one more question -  your Photos Library should be about the same size as your iPhoto Library.  Did you delete many photos from the Photos Library?  Or are you using iCloud Photo Library?

I just want to be sure, that yu are now having really all photos from iPhoto in your Photos Library, that you want to save.

If you are using iCloud Photo Library with the option "Photos > Preferences > Download Originals to this Mac" disabled, forget what I said about the hard links between the libraries.  The syncing with iCloud is breaking the links over time, so removing the iPhoto library will free more storage than without iCloud Photo Library active.