According to Mosby's Medical Dictionary (Spanish-English with E-S/S-E Vocabulary), the preferred term is convulsión.
I would recommend getting a Spanish-English medical dictionary. Mosby's is one of the most commonly used in Mexican faculties of medicine. I bet there're PDF versions somewhere on the internet ;)
S.A, Diccionario Mosby (Inglés-Español, Español-Inglés) de Medicina, Barcelona, Elsevier, 1a ed., 2008.
