I was actually wondering the same thing, T8 or T5...
I'm building two new enclosures for my dragon, and like the OP, planned on doing 24" high, with a 24" T8 10.0 UVB. Every major build video on YouTube, or plans I see for a 4 x 2 x 2, or a 36" x 18" x 24" (LxWxH) all use T8s with a basic fixture from Walmart.
From all the info I was given and researched, you'd have to have your dragon pretty high up to get full benefit of the T8, within 6-8" like Cooper said, which is why I was also thinking of going T5 for that height. I may consider 18 or 20" in height now, and stick with a T8 10.0.