T5 is the best. T5 or T8 refers to the diameter of the bulb. T8 are wider. I've never heard of T12.
Perhaps you are confusing the numbers with the strength of the UVB output. Arcadia produces 6%, 12% (which I use for almost everything) and their new 14%. The higher the %UVB, the deeper the penetration in the cage. Whether 6% or 12% depends on your cage height and species. I would use a 12% in a normal cage and make sure the animal can get out of the UVB rays at all levels in the cage if it so chooses.
You mentioned in your first post about immediately taking your chameleon to the vet. I would recommend you not. Few vets can tell whether a chameleon is sick or not unless you tell them. When they are stressed--such as at a vet's--they are so defensive that even the best reptile vet in the world will not see illness unless the animal is on death's doorstep and even then they can appear surprisingly healthy and vigorous. Save your money. I do take my animals to the vet when they need them. Sometimes I do when I just want my vet's opinion on some concerns I have, but my collection is pretty much all wild caughts which have a host of problems that a captive born chamemleon won't have.
If you are getting a captive born and bred chameleon from a breeder, that is of a fair size and weight, they should never need a vet. Even if they have parasites, testing and treating is usually not appropriate when they first arrive. (The parasite load will increase with the stress of shipping, etc.)
