I think the best option is ''Spend the 50 at once, get 11 normal chances, and one 5* girl'', the motive is:
1-As far as i know you can get all(I'm not sure if you can get Lilac) the 4* girls in the normal gancha, in the other hand you can't get 5* girls at normal gancha.
2-As events go on you will have a full team of 5*, and 4* will be useless, to give you a example, i started since the beginning of the game and i already have 9 5* girls(1 guaranteed to everyone at the start of the game, 1 from a guaranteed gancha at the first week of the game, 6 from events and 1 from Anime Expo code) so it won't take much time before i have a full team of 5*.