I've been very happy with my CB2000, I've had it for about 5 years or so, it was the first chastity device I owned. Recently, as in a few months ago, I bought a CB6000 and have been wearing it enough to get the fit right and to become accomplished with putting it on, etc. I go back and forth between the two devices, and I think that's because there are pros and cons to both. I think I've been wearing the CB6000 long enough to have a few (hopefully) helpful comments.

First, I don't believe there is one "best" device for all - there is a best device for you, based on a lot of factors, not the least of which is your anatomy and the level of importance various aspects are to you.

I'm a "grower", which basically means that flaccid my penis can get short and small, but erect it's right in the middle of the average range. I'm older (60's) which means my skin isn't near as elastic as it once was. All of these are important considerations for the safety and comfort of the device you choose. I'm typically unlocked every evening, though not for orgasm, and often not even for teasing. This helps avoid some of the problems devices can cause in long uninterrupted wearing, though I have worn the CB2000 for as long as 3 days without a break. Based on my experiences with both I can extrapolate what long term wear in each would be like for me.

I like the open bars of the 2000, it's easier to make minor adjustments in position of the penis when peeing (which helps avoid splatters and sprays) and stays cleaner during a day's wear. Plus the caged look is appealing. On the down side, changes in size during a day can and often does end up bunching up some skin either just under the penis head, frenum area, and/or near the base. A little swelling or edema was the result and this isn't good, and even started to feel a little numb sometimes and that's a good reason to get unlocked and let things get back to normal. It was a little disturbing that this could happen in as little as 6 hours.

So that's when the idea of the 6000 seemed like a good solution, it's design would essentially allow the penis to be pulled into the tube and the narrower shaft would avoid places for the skin to bunch up by holding it in place. I was also attracted to the cuff ring closer, which makes it possible to get off if necessary when everything is engorged without trying to push a swollen package through the solid ring. I was concerned about the enclosed design which could mean that if the penis were shrunk back from the slit, pee would fill the head of the device before running out. And it does.

Pretty much everything went as I expected. Here's the run down.

Putting on the device
Cuff Ring

CB2000 - sometimes difficult, even with experience, a lot depends on whether they are hanging low that time or not. Nice to have on even without the device as erection enhancer, but hard to get off if nice "blue" balls swell comes about under long teasing and edging.

CB6000 - not as easy to assemble the pieces on the privates as I thought, and I'm often leaving the cuff ring intact and slipping into it as if it were a single piece like the 2000. But it's nice to think that if I needed to get it off, I wouldn't have to try to slip through the ring. Because the posts stick out from the cuff ring it isn't really as useful as a cock and ball ring, like the 2000.

Cage Assembly

CB2000 - I got to where this was almost second nature and found that the slots and gaps made it pretty easy to get in.

CB6000 - Just switching which piece the pins were attached to (cuff ring versus cage) and which piece the holes were on turned out to really require some relearning. But beyond that, it's necessary to use the "sock trick" to pull the penis into the cage. The stocking has to be thin enough, and not too far down the penis, and not too little, and sometimes it's harder to pull through than others. This definitely takes longer and is more trouble than the 2000. However, it does all I hoped it would in terms of minimizing edema.

Pissing and Cleanliness

CB2000 - a shower gets water into virtually all aspects of the cage and toilet paper folded easily dries between the bars if alignment wasn't quite what you thought.

CB6000 - frequently the head of the tube fills with pee and then runs out over my ball sack. The cage has an odor at the end of the day since toilet paper can't get very far into the slit in the head, and the penis seems to sweat in there sometimes. (Yes I've seen the little squirt bottles etc for shooting water into the head etc for cleaning while wearing - doesn't impress me much, especially as a solution for during the day every time I go to the bathroom)

Appearance and Other Indiscreet Issues

CB2000 - The cage is a little bulky and even though I don't wear tight pants, laying on a massage table (for a legitimate therapeutic massage) I suspect it looks odd under the sheet, and of course the lock moves and clicks, which can be fun in a risque "will someone wonder what that is" sort of way but also can be concerning - again depending on context.

CB6000 - the lock fits tighter and doesn't move so the noise factor is eliminated and the natural outline makes it (I think) less noticeable. (And yes, like a lot of CB wearers I have had the thought that it would be interesting for someone, especially a woman, to notice and ask, but that's mostly fantasy)

Longer Term Wear

CB2000 - I don't see this working for me for long uninterrupted wear. Aside from the edema issues, which can be serious, as good as I had the fit long days invariably resulted in chaffing and/or pinching.

CB6000 - I've had none of the chaffing or pinching issues with the 6000 and except for the cleanliness aspect, feel it is more comfortable and suitable for longer term wear without interruption.

So that's my take on the two devices. Since the 2000 has been discontinued I suspect that folks entering the scene have fewer options - unless they go with one of those 2000 knock offs, or find some old stock, or a used device.