I should have checked this before I posted about the M3, but I just checked and Horenstein's Nielsen's 5th is also available (for $4.99). I'm not certain what's going on here. Looks like the original Unicorn-Kanchana licenses might have expired and somehow iTunes has acquired the recordings. Whatever, the reason, it's nice to see these recordings being made available after so many years of being out of print (although I do have the CD of the Nielsen).
Probably because of the difficulty of acquiring these recordings, they have taken on a somewhat cult status. Although Horenstein's M3 is not my favorite, it wasn't when it was originally released in the '70s, but it is still among the better recordings. The Nielsen, however, is still my favorite recording. I have several recordings of the Nielsen 5th, and it is Horenstein that I think that follows the composer's instructions in the first movement and lets the snare drummer do just about anything short of throwing the drums on the floor and stomping on them in his attempt to stop the orchestra; I particularly like the rim shots.