Well i couldn't manage to chose wich one to start with so i read both plots and then choose Umineko
Even though I know that summarys are not always representative of the anime ^^
But if i like it i'll definitely watch the other one as, as u said, they are from the same series !
Just for culture, this series is based on the visual novels of Ryukishi07. The series is "When They Cry" and for now there are 4 chapters:
Higurashi No Naku Koro Ni: When They Cry 1
Higurashi No Naku Koro Ni Kai: When They Cry 2
Umineko No Naku Koro Ni:When They Cry 3
Umineko No Naku Koro Ni Chiru:When They Cry 4 (this one doesn't have an anime for now)
Of course is ongoing.
Note that Higurashi have 2 OAV series: Higurashi No Naku Koro Ni Rei and Higurashi No Naku Koro Ni Kira.