With that thought in mind, let me say that the key to keeping your self chaste - meaning, that you can't get yourself out of the device is to limit the distance between the retaining ring and the tube. The size of the retaining ring is irrelevant to keeping the balls trapped, as long as the larger size of the ring doesn't increase the open space between the two parts.
With regard to my jailbird, I chose to follow the advice of one poster -'pansy tart' who talked about how to minimize the size of the tube and still retain comfortableness. If interested, read the 11.28.11 post. He describes measuring ones' penis when it is in its smallest state and then subtract a 1/2". I did that and use a Jailbird cage that is only 2.5" long but much larger in girth than the 6000 tube. The problem I am having with the Jailbird is fluid retention in my scrotum because the retaining ring is too small. I'm going to try a 1/4" larger ring. One might not think that a 1/4" increase will do much but after figuring out the area of a circle, I learned that I am increasing the area within the ring from 2.5 square inches to 3.5 square inches - a pretty significant relative increase in area. I'm hoping it solves the problem.
The differences between the two devices are not great. One significant difference is weight. The first time you fasten yourself into a steel cage after having lived inn a plastic one, you can't help but notice the weight of the steel. It's significantly more. It's not uncomfortable but it is definitely heavier.
Second, I've found the thinner steel ring to be more comfortable than the thicker plastic retaining ring of the 6000.
Third, I love that I can let my penis breathe in the jailbird. I was hesitant to increase the tube diameter to one larger than the 6000, but I selected a tube ring size that will slip over by cock with ease. As a result it's not squished inside and up against the plastic tube the way I fit into the 6000. For me, the only way I can get myself fully in the 6000 tube is to either apply a lubricant to my appendage to decrease friction and try to push it fully inside. It never makes it quite all the way in but will settle in eventually. My preferred method is the 'panty hose' method in which I slip the open end over my penis and push the other end in the tube and out the pee hole. If I push the head of my penis (that is now covered with the panty hose) in the tube and pull the other end, the stocking will pull the penis completely in. I just keep pulling on the panty hose until it eventually pulls free and out through the pee hole. I fit inside all nice and cozy this way without any feeling of confinement or crampage of skin that sometimes happens when I use the lubricant method.
For me another advantage to the steel Jailbird is the ability to be resized. Now granted, the 6000 comes with resizing ability right from the get-go. You have a variety of retaining rings and spacers to choose from. Before purchasing a Jailbird, I recommend purchasing the plastic rings. You can choose a cage size, as well as a retaining ring size. Don't be afraid to go larger with either. The key is comfort as you may end up spending many weeks if not months trapped inside the cage you so dearly want to have your wife agree for you to purchase. Choose your fate wisely and think comfort. That is what I'm trying to resolve fully, even after having lived in the jailbird for over a year.
One of the positions that sometimes cause me discomfort is sitting. The 6000 is made to keep the device a low profile one by pointing the penis tube almost directly downward. The problem comes when sitting as the tube touches the seat and is pushed up - really the stress is an outward (erection direction) motion that can cause the skin underneath to stretch and become irritated. I rectify this when driving long distances by sitting on a backpacking pillow to add just an inch of lift so I can sit comfortably. My Jailbird doesn't have this issue because the 'tube' is so small. At 2.5" it stays out of the way and I can sit comfortably without the need for a booster pillow.
Looks is another consideration. Katie likes the look of steel and for her, she prefers me locked in steel than in plastic.
Durability: I have broken two retaining rings that have required replacing. If parts do break, you can order others through the company but the initial cost you pay for the 6000 may not be your last expense.
Guarantee: If some part of the 6000 breaks, and it will with time, you are on the hook for replacement parts. The Jailbird is guaranteed for life. I like that.
The lock. The lock on the 6000 'clinks' when I walk quickly. I have to put a rubber band around it to keep it from making a methodical clunk, with every step. On the other hand the Jailbird lock sticks out to the side and doesn't sit nicely with a low profile on the top of the cage. My solution.... I purchased a tap and die set and threaded the cage and retaining ring holes so that I could put a screw in it. I use a 1/2" screw that fits perfectly. I love it and it's secure. You can purchase screws from the company with some odd shaped female head that only your keyholder's 'screwdriver' will work in it, but for me a simple Philips head with the promise that I won't unlock unless I ask, works fine for us.
That's my comparison of the two. Love to have others react and comment if you wish. I know this is a topic of interest among newbie's to chastity so feel free to share your thoughts and knowledge for the benefit of others. This post is but the opinion of one. Love to have others chime in.