I've been told that if the TA is a PhD candidate and has been a lecturer a few times, then it's not as bad, but still not as strong as coming from a professor.
Been a while since I have seen this question, but have a very good answer for you. Have the TA write the letter, but have it cosigned by both the TA and his/her PhD supervisor. In the end, it is better to have a PhD professor who has been teaching for a long time, but if there is difficulties in obtaining a science letter this would be the best way to go about it.