Well, even if it can't technically connect (very close), using your dj can be extremely risky in that situation. Marth's ground mobility >>>>> aerial mobility, the marth on the ground can shark the **** out of you. You can get UPthrown (low%) and eat the utilt but DI it behind and get away (still have dj).
Against floaties, Marth's follow-ups from a throw are extremely limited (against fastfallers, uthrow is clearly superior), so the strategic advantage you could gain from a fthrow (ex: fthrow them closer to ledge) is actually quite useful, even if you don't get a guaranteed dtilt/ftilt/fsmash out of it. This essentially resets neutral even if you go for the EZ follow-up and whiff, and Marth's neutral is sick.
Just because uthrow doesn't 0-death floaties like it does spacies doesn't mean it can't put them in a horrible position. Problem is, it takes a bit of effort to continue uair'ing him until you can KO him. Also consider marth's "problem" getting ko's at high percent. It can make you wish you fthrew him closer to the ledge, baited/punished one oos option, and edgeguarded him (sometimes).
There are many factors. In general, uthrow is probably better. Without it marth wouldn't be top tier.