I just got one of these and used it for the first time on a four day trip in the White Mountains. Mine is the Hennessy Expedition Asym with the velcro bottom entry. I loved using this thing! Far more comfortable than any tent, easy to set up and take down, and super compact.
The advantage of the zip version is that you can lie in it like a conventional hammock with the bug net retracted. You can also lie in the bottom entry version like this if you fold it a certain way. Also, it would be easier to arrange your sleeping gear in the zip version before you climb in. The disadvantage of the zip version, as I see it, is a potential point of failure if the zipper breaks.
I did not mind using the bottom entry and appreciated its simplicity. If you really can't decide, get the bottom entry and if you don't like it, have someone add the zipper. There are various folks on Hammock Forums who do this mod for people.