AG-Grid has some features that tabulator lacks, like multi-cell editing and cell group selection. (but these features will be coming soon to Tabulator)

AG-Grid focus's on being a fully functional spreadsheet where as tabulator goes more down the route of interactive table.

Tabulator has a publicly visible development roadmap so you can see roughly when new features are going to come online.

There is a new release every month or so, so if there is a feature you feel you need you can create an issue on GitHub to get it added to the roadmap.

Most importantly, Tabulator is FREE even for commercial use!