My vote would be the dump trailer.
I own a dump trailer and a C5500 flatbed dump. Also own a 3500.
The C5500 like the F550 is a 19,500 lbs GVW truck. The C5500 for me weighs in at 12,300lbs with tools, gas, 14' flatbed dump, and two guys. That leaves 7,200lbs of "legal" payload. A 14,000lbs rated dump trailer will be about 4,000lbs which gives us 10,000lbs of legal payload - More than a class 5 dump.
BUT, my C5500 gets 6.5MPG. Thing doesn't care if it is empty, full, city, or highway... The MPG is 6.5 (gas).
However a class 3 truck can legally pull the 14,000lbs trailer, and while MPG will drop when loaded you have the option to get better than 8-12MPG unloaded. Lowest the 7.4L 3500 has done is 6MPG loaded and working hard.
The dump trailer give me the option to use a lighter duty truck that gets better MPG, lower parts cost, and seems less repair cost too compared to a medium duty. All I got to do is load differently.
The main compaint I have about dump trailers that is not to be overlooked is many have perminate sides that do not remove. Some have folding sides which is what I would get. That way you can unload mulch from both sides instead of only from the rear or liftin every scoop above the formed metal sides.
Even better would be a flatbed dump deckover trailer with stake pockets to utilize wooden sides of 2.5'-3' widths. Deckover wheels would allow tool boxes for the ultimate kickass dump trailer being hauled by a truck you probably already own.
Another advantage with a dump trailer is if you find you need more mulch, drop the trailer and use the truck to pickup the other 1,2,3, or 4 yards while the guys continue to work.
A disadvantage of a dump trailer is the skill level required to use a trailer. If employees will be using the trailer, are they smart enough to back in into a drive if need be without damaging turf or a mail box? The nice thing about the truck is ease of getting prduct there with less intellegence/motivation.