Sealed hammock:
chrysalis hammock
Personally I can see no advantage over a tarp. For one thing a tarp affords you space to work, cook and sit under if the weather is poor, as well as somewhere to stow your gear and dry wood. A sealed hammock will have reduced airflow which means condensation in the summer and ice in the winter where as with a tarp set up it's a lot less of a problem, as well as meaning you can set up in different patterns, including 'porch mode', which means it's more flexible to the different seasons, especially when you can add doors to some tarps (or rig it with ponchos). Plus if your weather protection is seperate to the hammock it means you can set up one without the other. And a combination of the two is lighter than both together, especially if you have multiple tarps.
Saying that, there's a thread over on hammockforums with a dude who strips down the chrysalis (as it's a bit of a pig to begin with in terms of weight) and ends up with a cool cat cut bridge hammock.