Single Layer vs. Double Layer. As seen in the ratings, the DL's can handle more weight than the SL's, but the DL's also give the ability to place a sleeping pad between the layers. This keeps the pad from shifting on you as much and is more comfortable than laying directly on the pad. Pads are somewhat unruly under your torso and placing them inside the double layer generally makes them easier to sleep on. This is the way to go if you plan on using a pad as your primary bottom insulation.
The space between the layers is the same size as the hammock body itself and can handle just about any sized camping pad. Some folks like inflateables, and many find them more comfy after letting much of the air out. Many might use them simply because that's what they have after switching from a ground setup. Some find them completely adequate or even preferable, while others feel like they are always rolling off them in the hammock. Another problem sometimes encountered with inflateables is that they generally aren't wide enough to cover your shoulders (more important in colder weather) as you may need coverage up to 30"W for full coverage. One option is the spe or (segmented-pad-extender)
Lots of others prefer closed cell foam (ccf) pads which generally have a higher warmth-to-weight ratio although they are usually bulkier when packed, but tend to say put better in the hammock. These are available on the roll so you can buy what you need and then cut it to your desired width/shape. If the foam is high quality, 1/8" can be enough for summer nights, 1/4" thickness for milder weather (down into the 40's), 3/8" for cooler weather (to freezing or below) and 1/2"-5/8" down into the single digits and below. One example would be to have a 1/4" piece and a 3/8" piece. These could be used separately depending on the temps, and then could be stacked together to get 5/8" thickness for the coldest of weather. If you camp mostly in warmer weather, combining 1/8" and 1/4" might be a good option.
ccf is available at a reasonable price and by-the-yard at many upholstery shops. I've found very high quality foam on the A1 Foam and Fabrics Website. It is on a 60" roll, so be aware that you'll need 2 yards to get a 6' long piece.
The mini-cell foam here is about the same weight but seems like it might be slightly more warmth efficient, but at a bit higher price. I found the 1/16 mini-cell skins to be more like 1/8 thickness
Many folks dislike pads and instead use underquilts as their primary bottom insulation. For more information on under quilts see Under Quilts 101
Choosing the Right HammockUnder 150 LBS:
Single 1.1, single 1.7 and double 1.1 Blackbirds, or single or double layer RidgeRunner. You're a lightweight, and any of these will do you well. Remember a double layer is highly recommended if you plan on using a full-length pad as your primary bottom insulation.150-175 LBS:
Single 1.1, single 1.7 and double 1.1 Blackbirds or single or double layer RidgeRunner. In this weight range, you may notice a slight loss of flatness by going with the single 1.1 Blackbird, but you're still well within it's weight range and that can still be a great option if you're looking to cut a lot of packweight. In contrast, the single 1.7, and double 1.1 Blackbirds are going to provide maximum support and comfort for your weight, the double 1.7 Blackbird is overkill for you and you won't see any increase in comfort by stepping up to that model. Single or double layer RidgeRunner models are also appropriate for you.175-200 LBS:
Single 1.1, single 1.7, double 1.1 Blackbirds and single and double layer RidgeRunner. If going with the single 1.1 Blackbird you'll experience a noticeable loss of flatness and support due to stretch and it's only recommended if you're very concerned about packweight and willing to accept some tradeoff in comfort. Single 1.7, and double 1.1 Blackbirds will all give you maximum support and comfort. Double 1.7 Blackbird is overkill for you and you won't increase support or comfort by going that route. Single and double layer RidgeRunner is also appropriate for your weight.200-225 LBS:
Single 1.7, double 1.1, and double 1.7 Blackbirds or double layer RidgeRunner. Single 1.7 and Double 1.1 Blackbirds will provide you maximum support and comfort. You can also step up to the double 1.7 blackbird if you want but there will be little gain in comfort and support by doing so. Double layer RidgeRunner is appropriate for your weight as well.225-250 LBS:
Double 1.1, double 1.7 blackbirds and Double layer RidgeRunner. Choose double 1.1 Blackbird if looking to go lightweight. In this weight range the double 1.7 Blackbird will provide you slightly more support, choose double 1.7 Blackbird for maximum comfort and double 1.1 Blackbird if you're weight conscious at all. Double layer RidgeRunner is also appropriate.250-275 LBS:
Double 1.1, double 1.7 Blackbirds You can certainly still go double 1.1 if you're looking to cut packweight, but in this weight range the double 1.7 Blackbird starts to be noticeably flatter and more supportive. Double 1.1 is recommended for those looking to go lightweight, double 1.7 recommended for those looking for maximum comfort. Double layer RidgeRunner is only rated to 250 LBS.275-400 LBS:
The double 1.7 is the hammock for you.