Well the like all important questions, the answer is it depends.
One type of swing is not better than the other, but they both have their advantages. Personally, I would recommend using a dual hook swing whenever possible. There are some key advantages that the dual hook swing has over the single hook version.
When you are using a single hook swing, there is usually a spreader bar that is used to set the width of a sex swing. On most swings, this is about 20 inches wide. This width is fine for a lot of people, but for some people, especially those with wider or bonier hips, this may be very narrow. When you sit in this swing, it will squeeze your hips and put pressure making it a little less comfortable.
When you are using the dual hook type of swing, you can set the hooks at the width that you prefer. This could be much wider like 30 or 36 inches. This will make the swing much more comfortable for a lot of people.
Another advantage of the dual hook sex swings is the weight limit. By having dual hooks rather than 1, you double the weight limit on your sex swing. Additionally, if you were to mount the swing incorrectly and it was to fail, the hooks would pull from the ceiling, and you would fall, but you would not have the large metal spreader bar falling on you. The hooks are much smaller and would be off to the side of you and most likely fall harmlessly to the floor rather than hitting you directly with a 8-pound spreader bar.
So from a safety point of view, dual hook swings are stronger and safer than their single hook versions.
One more significant difference is the price. The dual hook sex swings can cost less than the single hooks because there is less hardware on these swings. While there are two chains and two hooks, there is no overhead spreader bar. So the cost can be significantly different.
But it is not an easy choice. 1 hook swings do have a few advantages. One major advantage is that there is only a single mounting point. This means that you only need to screw one hook into your ceiling rather than 2. This is imperative to some people and should be considered. Another advantage of the single hook swing is that it can be made into a spinning swing. You can not do this with a dual hook swing. For me, this is not that exciting of a feature, but for some people it is. Lastly, depending on the position you use your swing in, the spreader bar can make a good handle and is easy to hang onto. The spreader bars are only on swings that have one hook and not in those that have 2.
So both types of swings have advantages. In general for comfort, safety, and price, I would recommend a dual hook swing whenever possible, but either may be suitable for your situation.