Do know the bottom shelters have an advantage---that non-removeable but netting has no zippers to fail and the bottom entry hammocks are usually lighter.
The bug netting will matter for someone in the deep south and obviously maybe not so much in the north....that's why in the mid lattitudes i alternate between Hennessey and Speer depending on anticipated temps.

There is something else the newbi needs to know about hammocks and that is cold-shoulder-wrap. The hammock (either type) will compress against your shoulders if you are wide enough (most men, and maybe not with some small women/kids)...some big people even get cold-hip-wrap too. So if you are only using a sleeping bag it will get compressed at the shoulders and you will get cold spots.
OK, so you want to use a thermarest pad....most people will find them 1. not wide enough to prevent shoulder wrap and 2. too easy to slide off of in the night....enter the underquilt.
Underquilts have obvious advantages. 1. they compress to small parcels and can go into the pack 2. they can offer more insulation than a thermarest but they have a downside (no pun here) too: 1. costly 2. hard to go to ground with 3. assembly time (though for me the underquilt is essentially permanently attached to the hammock, it stays on when i take it down, so on multiday hikes each night its already attached and dialed in.

Now for the survival minded you can use a taco...a piece of breathable ripstop will do, that acts like an underquilt but it has no insulation--you provide the insulation with expedient material e.g. leaves/pine needles/your clothes/ etc.

OK so you go with an underquilt-doesnt that make the sleeping bag moot? well yes it can, that is why i use top quilts, the ones i use are made by Nunatak Gear but Jacks-R-Better has them too....or you can take an old sleeping bag and make it into both bottom and top quilts--just a bit of cutting and sewing required.

Then there is the Pea Pod which as mentioned in previous post rules for winter use.

Hammocks do have other uses besides shelter....that same bug netting makes a great seign (sp?) for fishing, the hammock also makes a great stretcher.

There are many more options in hammocking...for example the pad extender by Speer, which makes the typical thermarest wider and is a cheap option but does add bulk to your backpack.