By the term "hammock tent" I am going to assume you are talking about the hammocks with a tent style canopy/screen attached as an all-in-one unit. I've seen them here and there at online shops, but they just don't appeal to me due to the lack of versatility. A hammock can be set up alone for a quick seat or mid-day nap. A tarp can be set up quickly for waiting out a rain shower. Put them together and you have a nice dry place to sleep for the night. The combo designs seem to only be good to sleep in, and appear to be a bit more complicated to set up than a simple hammock.
I have a few hammocks, but my favorite is the Hennessey Explorer Asym Zip. I always sleep great and don't wake up stiff and sore. I have the larger hex rain fly, and usually set it up in what I call "porch mode". Stake one side to the ground against the prevailing wind, and raise the other with hiking poles or sticks so I have a nice wide-open view.
If you've never slept in a hammock, I would recommend a few test runs to make sure it is for you. Also, hammocks sleep colder than a tent with a pad. Anywhere you compress your insulation will feel colder. I am cold natured, so temps below 60 degrees I use a 0 degree bag and I'm warm and toasty.
As far as just using a tarp and sleeping on the ground.....have you heard of ticks, chiggers, snakes, centipedes, millipedes, mice, lizards, spiders, moles, skunks, toads, caterpillars, crickets, cicadas, raccoons, and opossums? I'd much rather be hanging above all that looking like a big fat taco for a bear!