There really isn't that much of a difference between the two. T5's are the latest thing, therefore have the most "buzz". Both are rated for tanks between 18-24" tall. T5 lamps last a little longer than VHO
, but generally
T5 set ups cost more initially to buy. So the long term cost difference levels out some.
As far as the clam goes it depends on which type you plan on getting. Some require medium lighting and some high. The high light species, like Crocea and Maxima, are recommended by some vendors and aquarist to only be kept under MH
lighting for long term positive results and proper coloration. Although some aquarist have been successful w/ fluorescent lamps by placing the clam high in the aquarium and having a high WPG