Phytoplankton is a necessary nutrient for your system as a whole. Most corals will benefit from some phytoplankton, but most important are the other creatures that you don't directly feed, the ones that make your live rock and
sand beds interesting, you know, the little sponges, featherduster/worms, those really cool things that you have no idea what they are but look cool (the stuff that people without tanks don't see)they need to be fed also.
DT's seems to be a good choice, also Golden Pearls seem to be rising in popularity.
I'm sticking with these two until Dr.Shimek's test results are all in.
HTH Phill