"T" refers to the diameter of a fluorescent bulb (in ⅛" increments)—not its output/power.
Either T5 or T8 are acceptable, but most seem to be gravitating to T5s; they're a newer tech that puts out more UVB more efficiently, and last ~twice as long as T8s.
That said, power would depend partly on the size/tech bulb you go with.
In most (not all) cases:
For T5, a Reptisun 5.0 or Arcadia 6%
For T8, you might kick that up to 10.0 or 12% respectively.
I think most here would recommend a T5 Reptisun 5.0 or Arcadia 6%.
The fixtures for T5 linear tubes are mostly made by the same company regardless of who sells them, so shop for price.
https://www.chameleonforums.com/threads/need-a-better-uvb-light.184148/#post-1689350
Yes, for most reptiles, linear is the best choice for many reasons.