Flex duct is easier to seal and usually performs better thermally.
Sheet metal will pass more air with the same static pressure. Flex
duct is quieter.
It is a trade off, depending on your situation.
With sheet metal, to have a tight system, you must seal every gore on
every elbow. You must seal every joint in the duct, plus you must seal
every longitudinal seam. Then you hust do the same thing with the
wrap. And if you install the wrap tight so it looks nice, the actual
R-value is about 1/2 rated value.
With flex duct, you just have to seal the connections at the ends, but
it must be supported at closer intervals with wider straps. But you
get full rated R-value for the entite length.