either will work if they are mastic sealed.

flex has only two areas that leak..where they
connect to take offs at plenum (take offs leak too)
and where the flex connects to supply boxes.
if you mastic seal these connections (and duct
take offs at the plenum) the will not leak and
condensate.

hard pipe ducts leak at every seam and joint.
if you take the time to mastic seal all of these
areas prior to insulating then hard pipe is a good
duct system. problem is..folks don't mastic seal.

hard pipe ducting is more expensive than flex.

I like flex..seal it up, strap it up and its a good
duct system.

hard pipe is also good, just more leakage areas.

hvac folks either love flex or hate it
same with hard pipe ducts. usually they install
the brand they love..LOL!

and remember the mastic sealing is in the
details. use paint mastic and/or hardcast brand
1402 mastic tape.
to have a properly sized & designed duct system
is a wonderful thing. pair it with a properly
sized hvac system & you are golden.

best of luck.

I've used both but since most of the installations or additions I was working in were in existing construction, flex was the only way possible to reach some areas. Just remember to use a lot of hangers or other support so it doesn't sag over time and cut down on air flow.

You want the lowest mass possible fort ductwork in unconditioned space.

When the system is not running, the ducts are going to start to warm up (insulation can slow warming, but will not stop it).

When the system comes back on the now warm ducts are an additional load on the system until they are cooled back down.

Less mass, less load.