I own both the Virgin and BSP sets of letters. Although not a definitive assessment, from what I can tell (and as Bukfan noted), they have the same content. I think Selected has two meanings here: 1) these obviously don't represent all of Buk's letters, and not all of his in print (e.g., Purdy, Martinelli); and 2) there are four Virgin volumes and 3 BSP volumes, so the contents cross-over a bit.