If you are considering a device for the first time, the choice that we recommend is the CB family of products. They are the easiest for most men to get on, and with the variety of rings and spacers included in a set, it is easiest to find the most comfortable fit.
If you prefer a silicon model as a first experience, Bon-4 may be worth considering because it does come with multiple rings. However, Birdlocked is the more popular model in this space, especially the Neo. Dick Cage has an advantage over Birdlocked in that if you get the ring size wrong, you can purchase a different sized ring.
Men who have a high ball sac may have difficulty with the silicon models. The width of the ring on a Birdlocked model is 30mm, and on a Dick Cage it is 35mm. If you don't have enough slack in your scrotum, it will be very difficult to get on, stay on, and wear discretely and comfortably. The width of the CB rings are only 12mm, and consequently are much easier to wear comfortably.
CB Models
The first consideration is the size of your flaccid penis. The CB-6000 will accommodate a 1 3/8" diameter and about five inches in length. CB-3000 is slightly shorter, but slightly wider than the CB6000. If either dimension are greater than that, the Curve may be the better choice. There is also the CB6000-S model, which is an inch shorter than the regular CB6000, and has been very popular since it's introduction.
Another consideration is appearance. While all models are pretty discrete under loose-fitting clothing, on most men the CB-3000 and CB-6000-S will be the one least likely to be detected. The CB-6000 is also quite discrete. The Curve does protrude out further than other models, but if you are large enough to wear this model, chances are that the bulge in your trousers has been noticed before. :-)
Comfort is another consideration, though they are all about the same once the right fit is found. Generally, people that have bought and used the CB-6000 have commented more about the comfort than the other models. Opinions vary though.
Silicon Models
Birdlocked models are all one piece, and have six models that come in a variety of colors and sizes. Since its introduction in 2010, the Neo model has been by far the most popular for two reasons primarily:
The ring design accommodates a range of 45mm to 52mm, somewhat self-adjusting.
It is designed to be more secure (i.e. more difficult to get out of) than earlier models.
Dick Cage was introduced in 2011, and does a very good job of trapping the penis to prevent escape (though none of the models are fool-proof). Initially, it was only available in clear, but is now also available in black. It offers a choice of three ring-sizes and two lengths of cage.
Bon-4 comes with multiple rings to help find a good fit. Like Dick Cage, it has a choice of short or long tubes. There is actually the "Plus" model which contains both tubes. The biggest criticism of the Bon-4 seems to be around the level of difficulty of getting the unit one, as the attachment of the ring and cage with the provided plastic strap can be very difficult to do without assistance.