Under what circumstance I should use FlexTable instead of HTMLTable?
You can not compare like that.
As per Docs
HTMLTable contains the common table algorithms for Grid and FlexTable.
So Grid or Flextable
extends HTMLTable
.
And
A Flexible table
that creates cells on demand. It can be jagged (that is, each row can contain a different number of cells) and individual cells can be set to span multiple rows or columns.
Now we can have a question Grid or Flextable
???
Here is the Performance difference between grid and Flextable.