Ok for me personally if I was using an enclosure with a mesh top (which I never would but that is personal choice for a number of reasons) and I wanted the UV tube to rest on top of the mesh then I would definitely go with a T5 tube rather than a T8.
The reason for this is the T8 tubes (which are older technology) produce UV light with shorter wavelengths which means that they have a shorter penetration into the viv. T8 also have a lower UV output than the more modern T5's which again directly impacts the amount of UV light that your dragon is exposed to. T8's also have a significantly shorter life span than a T5 meaning that their optimal output of UV light is only about 6 months before it needs to be replaced whereas a T5 normally lasts for 12.
The final factor you need to take into consideration when placing tubes on top of the vivarium mesh is the mesh itself. The mesh acts as a barrier to how much light is penetrating down into your viv and as an average indicator it is normally accepted that about 30% of that penetration is lost (though this will definitely vary from brand to brand).
The recommended distance from tube to basking spot, to allow optimal light absorbtion for your dragon, with a T8 tube is about 8" with a T5 it is 12-15"
So taking all these factors into consideration (height of tube to basking spot, intensity of emission, reduction of light penetration through mesh, etc) it then becomes apparent that a T5 is going to be a much better option for an 'over-mesh' set up.
In regards to the question of brands and why some are recommended more than others boils down to the manufacturing process of the tubes themselves and the actual amounts and frequencies of the UV they produce. UV light, like all light spectrums, is broken down into frequencies and natural sunlight contains various percentages of these frequencies within its make up. So whilst two seperate brands may be emitting 10 or 12% of the total UVB spectrum in their overall light emissions one of them may be designed to better match these frequencies and percentages to that of natural sunlight than the other.
To complicate matters more UV light is further split into two categories - UVA and UVB and the percentage emitted by tubes of these two types of light can also vary widely between different tubes. Again you want a tube that matches this split to natural sunlight as close as possible.
Both of the brands that are recommended on this forum have been thoroughly tested and have proven to match natural sunlight as close as possible, both in the frequencies of light they emit and in percentage ratio too. A lot of other tubes sold as UV reptile tubes do not have this match and some have even been found to contain dangerous levels of various spectrums and frequencies of light!
An easy way to visualise this is to put two different brands of daylight bulbs side by side and switch them on. You will notice the colours dont match. This is down to their manufacturating process which results in them both emitting different percentages of the various frequencies and spectrums of light.
In regards to why your UV tube has a bearing on the colour of your dragons poo is simple to answer. The efficiency of your dragons digestive system has been fine tuned by nature over millions of years to be controlled directly by the UV emissions produced by the sun. A dragon uses the absorbed UVA to maintain its digestive system and the absorbed UVB in the metabolization of calcium and D3 (used for growth and bone development) If the bulb you are using is not producing enough of the correct UVA light frequencies then your dragon might not be fully digesting the food that it eats - which in turn would have a direct influence on the colour of your dragons poo!
Hope this has helped somewhat in the questions you asked
