What kind of cage isn't really important. The cage or enclosure is just to keep your dragon in one place and make it easier to provide the environment it needs to thrive.
For adult dragons the cage needs to have a floor space of at least 4'x2' (basically 8 square feet) minimum. Height can vary between 16" to 24". To hold in heat having 5 solid sides (bottom, back, two ends and the top, with screen front or all solid sides with vents or a partial screen top, or various combination).
Heat rises so an all screen top lets heat escape easier...which can make keeping inside temps difficult, especially if you have a cool room temperature (under 65F especially at night)(average room temperature is considered 68F).
A solid top allows mounting of light fixtures inside the cage which makes it easier to ensure UVB and heat reach the dragon and basking area. If screens, then you are looking at lights being filtered partially by the screen which reduces the amount of UVB and heat entering the cage.
Glass cage don't hold heat well but build up heat fast in direct sunlight. (hence never leave a glass cage in direct sunlight whether inside or outside the house. Just standing in the sunlight coming from a window you can feel the difference in temperature if you stand outside the area the sun shines in). Solid cages made out of wood, PVC, and other materials work good in keeping heat in and humidity. These type of cages need to have some form of ventilation, either vents on top or sides or back. Or screen doors.
Light colored cages are best as they reflect light and create a bright environment, which bearded dragons like. Dark colored basking structures absorb heat better than light colored structures.
Doors can be on top or the front. Front is better for bearded dragons for various reasons...it does make it easier to access the interior of the cage. It is also easier to handle and interact with your dragon as he is less likely to think the hand reaching for him is a bird. Birds attack from above. Moving towards your dragon from the side, is less threatening. Also, front doors make it easier to stack cages for obvious reasons.
For commercially made cages, most tend to have recessed areas for basking lights and UVB florescent tubes. But as these are still technically on the outside (as the dome light fits in the recess but there is a screen separating it from the interior), it isn't as effective if it was just mounted on the inside. This works for many reptiles that tend to jump high and more likely to get in contact with a bulb. Bearded dragons don't tend to jump up at lights so there is little chance they can get in contact with a hot bulb (unless it is way too low or the basking spot too high and just sitting up puts them in contact (sitting up being when the bearded dragon is pushing upwards with its front feet and head held high).
To decide which design matches your needs and budget, check out the various cage designers (check the business tab above and look under 'cages/supplies' and also the cages and habitat forum and the same category in the photo gallery. This will give you much more variety of cages and designs which you use to decide what is best for you and your dragon. Though many cages won't be made for dragons specifically, using the above guidelines (basically more floor space than height, keeping in mind dragons are semi-arid/scrubland and light forest areas creatures) it won't be difficult to modify or ignore the more arboreal type cages. (ie more tall than long, all screen etc)
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PHLdyPayne