One difference I believe is that Identify reads the whole image file into memory and then extracts the EXIF data. I don't think exiftool has to do that because it is only getting the EXIF/IPTC metadata.
I'm not sure how much metadata Identify understands. Exiftool is specifically designed to handle not only EXIF metadata but also IPTC and XMP amongst others and can set that data very easily too. It also has specific code to handle many manufacturers maker codes. It is very well tested and under continuous development.
I don't think Identify has the high level functions to set/get such a wide range of metadata. Geotags are another area that Exiftool handles that I'm not sure Identify does.
On the other hand, identify appears to be saying more about the image content (stats)?
UPDATE: You can see the purposes of IPTC and XMP easily enough using Wikipedia.
IPTC is a set of additional standard metadata originally used by newspapers. I use it extensively to catalogue my own images with author, copyright, geocoding, etc.
XMP is Adobe's extended metadata format, using embedded XML, for their products. It isn't as well supported outside Adobe products. Many free and lowcost tools as well as professional ones support IPTC. It is even possible to use BOTH together!!
UPDATE: In my view, IPTC is the best metadata format to use for information beyond the camera data. It is very widely supported both in free and commercial tools and recognised - even required - if you ever want to sell any photos.
Some useful Windows tools that handle EXIF and IPTC:
- Geosetter - This hasn't changed since 2011 but still works fine. It uses ExifTool which does get regularly updated & it detects updated versions of that. You can set any IPTC field with this though position and direction are its focus.
- IPTCExt - Although last updated in 2005, this excellent Windows Shell Extension gives you access to EXIF and IPTC metadata by right-clicking on an image in Windows Explorer
- Google Picassa - image catalogue software, free from Google. Understands some IPTC data and can set it too though you need to treat that with caution as there is some confusion about whether subsequent updates are saved in its database rather than the file unless you export the file again. I recommend doing some experiments for yourself.
UPDATE 2: I don't often use Linux desktop but here are some posibilities:
- DigiKam - probably the most commonly used tool.
- jBrout (Python)
- pyExifToolGUI (Python)
- XnView - only free for private/educational use
As always, WikiPedia has a useful list of metadata editors too though it is mainly Windows software.