Question: “Should I summon Skadi or Summer BB?”
In all seriousness, Skadi is a Big Four support, one of the most powerful servants in the game. Summer BB is good, and has a very interesting gimmick with her hand lock, but isn’t exactly a game-changer. In terms of pure power, Skadi is the obvious choice.
Moon Cancer is probably the least important class to own. Avengers aren’t particularly dangerous, as they deal and receive neutral damage against basically everything. You can just pick your most powerful servant (assuming it isn’t a Ruler) and wreck them. Regular BB will be returning next year, so if you really feel you need a Moon Cancer, you can get her then.
Definitely not. It takes about 300 SQ to reach a 50% chance to get a given rate-up SSR. So you’re probably looking at around $250 worth of SQ if you want a serious shot. SSRs are a major investment, and you should carefully consider before rolling for one.
Personally, Skadi and Summer BB are two of the only 4 banners I planned to spend SQ on, this year (the other two being New Years for Tamamo and Guda2 Poster Girl). I feel they are both worth the investment.