Right now, T5s are costlier than T8, but I think its just a matter of production volume.
It's estimated that a RE80 lamps cost about 28 cents a lamp in phosphor at production level. As the raw materials go up in price due to shortage of rare earth elements, there's definitely interest to reduce the amount of phosphor used per lamp and with T5s obviously having 5/8 the surface area of T8s, it maybe of interest for the manufacturers.
http://www1.eere.energy.gov/buildin...ial/pdfs/app_3c_lamps_standards_final_tsd.pdf
You most definitely shouldn't retrofit T5 into T8 using adapters, because the shorter length of T5s will cause the lamps to look like they have darkened ends.
For now though, I would go with T8s except for under counter lights where there is a great value in lower profile.
F54T5/HO is also catching on in high-bay, but they compete side-by-side with T8s and I don't have a definite opinion one way or the other. 32W T8s offer better system efficacy (ballast + lamps) although T5/HO system could fairly well have a better luminaire efficacy (system efficacy multiplied by fixture efficiency, which is lumens that leave the fixture per input watt) due to increased fixture efficiency through higher efficiency optics.