http://aster.ohmydays.net/pw/waravatar/#class=2My conclusion:
-Getting 6 different random S cards (one for each appropriate slot of course) will give a slight boost over Warsong City (S-A six set cards) and Six Candleflame Sovereign (full set A cards, AEU/EU bosses). However, since rebirthing/awakening a card requires the exact same card, it might be cheaper/higher possibility to just collect WS or AEU/EU set to get max reawakening with max stats you can get from the cards.
-No reason to keep B and C cards, unless you're on a low budget. They give worse stats at max rb2 level compared to lvl 1 S card non-rb.
-Combining 2 different sets of cards, for example: FB SoT and FB Aba set (3 of each) doesn't give the same stat as if you have a one A set that contains 6 cards. Look here folks.. The best set is the one that give the highest % set bonuses.