I believe both also from Arcadia, red aro hobbyists tend to use D3 previously for tanning due to the UVA and UVB value in D3 tubes (meant for reptile) before Arcadia developed Arowana UV tanning lights, if not mistaken they improve the percentage of either the UVA or UVB value... juz google and u should be able to get your answer....
if you want to tan, then UVA and UVB is key.
reptile bulbs are superior. arcadia tan lights (i hav eone) has a red colour and is artificial, hard to monitor actual development.
though in asia, its standard to use red 'lipstick' lights, so even silvers look red! even my buttocks which never seens sun is red thtas how red it is.
i would opt for reptile bulb, but if you buy the unit and you get a arcadia tan bulb free, use it and replace it after 4 months, its not gonna stain your fish or anything
According to their website,
Arcadia D3 UVA30%, UVB6%
Arcadia Arowana Lamp UVA27%, UVB5%
...so actually its a reduction, not clear on the reasoning but it did mention that the Arcadia Arowana Lamp was developed to tann red aro naturally but without harming it...