The difference is that tubal ligation is a permanent form of sterilization. There is a separate commandment, based on Leviticus 22:24, forbidding sterilization of animals as well as people. It is considered a violation of a Torah commandment to sterilize a man and violation of a rabbinic decree to sterilize a woman. Thus, a vasectomy is absolutely forbidden but, in very serious health situations, a tubal ligation would sometimes be permitted for women. The pill, on the other hand, is temporary and does not interfere with the normal progress of sperm into the female genital tract. It is thus generally the halachically preferred method of birth control when contraception itself is permitted.