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Both AbiWord & Gnumeric are designed with low resource consumption in mind. As such they are great applications for older computers.
Pro Great import/export features
Pro Free and open-source
Pro Straight forward
'Old' style menu structure is lovely to support users.
Over 30 languages are supported.
Pro Collaborative editing
Abiword is able to edit online collaborative documents.
Con Almost abandoned
Very few updates and bug fixes.
Con Default format is only readable with Gnumeric
Gnumeric is able to read/write a lot of file formats including ODS, XLS, XLSX, but it's default .gnumeric format is only readable with Gnumeric.
Con AbiWord has a dated aesthetic
The interface hasn't had a facelift since it's inception in 1998.
Con File interchangability
You may like it or not, but this is an O365 dominated world. And at some time you will need to send somebody the result of your work digitally. Then not being able to share a compatible file format is a pain. Even if your application is great and light (which it is).