Q: Number of photos in iPhoto and Photos Differ :/

A suggestion - do not delete the iPhoto library - there will be very small disk savings and you may need or want it - if it is gone then you have problems

as to space both libraries like to the same original photos so do not use much extra space - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

Migration does not remove dups - maybe those photos are corrupted but in general everything is migrated - and no there is no magic way to see what is missing

LN

Is it possible the 396 are just duplicates of some kind, and only one version of those migrated? Or some other explanation?

Are there RAW&JPEG pairs in your iPhoto Library?  Photos counts the pairs only as one photo.

In which view are you comparing the numbers? Compare in the "All Photos" album, so hidden photos will be counted. And compare the contents of the Trash in iPhoto to the Recently Deleted album in Photos.

When I delete or move photos onto another drive, should I do that from iPhoto of Photos?? I'm planning to move a lot of them off because my computer is VERY slow and I need to free up space.

If you are going to use the Photos Library now, move the iPhoto Library to another drive. That will save the storage used for previews and thumbnails in the iPhoto Library.  Keep the library as it is as a backup.

If you still need to free space, start to weed out redundant photos in your current Photos library.

First of all, when I back up my computer, does it not back up all my photos as well?

How are you backing up?

When you are are using Time Machine, it is perfect for restoring your computer as it is now, and Time Machine will also allow you to go back in time, but not indefinitely. Time Machine will delete older files from the backup, when it needs space for newer files. You may no longer be able to restore the iPhoto Library from your Time Machine backup in a few month. I think that it is good practice to have separate archival backups of all photo libraries before I start a major reorganisation of the libraries. You may accidentally delete photos and only notice it later, or bad blocks on your disk may cause a JPEG corruption. Such problems can go undetected for years, because the original master files in the libraries are only accessed when you edit or export photos.

I just had to open my backup copy of an iPhoto Library from 2008 to restore a corrupted JPEG:

Second of all, other than for "backup" is there any other actual purpose to have both iPhoto and Photos libraries??

Photos could not migrate everything. To be able to use your books, calendars, cards you need to open the library in iPhoto.

And some things simply cannot be done in Photos - just compare the functionality - see Old Toad's User Tip:

  Photos vs iPhoto:  Features and Capabilities

iPhoto is  still the more versatile application; I simply would not burn the bridges back to iPhoto until you are sure, that each and every photo migrated correctly and you are sure, that Photos will suffice for your needs.

For example, I discovered that the dates for photos taken in different timezones had been migrated incorrectly to Photos and some titles were missing. It was easier to fix this by keeping the original library open in iPhoto while I adjusted the photos in Photos.

You're welcome!