If you ever have to work on or modify the ducts, it is nice to have it
hanging. (But WELL supported.)
With a heat pump and R-6 ducts in the attic, here in South Ccarolina,
the heat loss in the heating cycle is 5%. The heat gain in the summer
(cooling) is 10%. Source: ACCA Manual J, 7th edition.
Hang it up so it can be serviced. Also, if you bury it, depending on
the vapor barrier, sometimes the ducts can sweat, which can cause
problems as well.
(Paul, I know there is a snide remark coming, so let 'er rip.)