for black ticket, choose your waifu, there are premium black units that won't be in chosen list, though
about summoning, spending 150 SC in a month get you a plat ticket, and I believe now is best time to roll with Lapis(just love her class) and Kasumi in first part then we have Nataku, Rion in second one, giving that you can choose Nataku or Rion using black ticket, going for first summoning or third for Seria seems reasonable.
but we are just in the beginning of month so using about 75 to roll what you like and save the rest in case we'd have item collection is what I recommend.