Choosing between CC BY and CC BY-SA. I think I'd vote for CC BY. While CC BY-SA is attractive in the sense that it alludes to a culture of "you take, you give back, we all benefit", I don't think that's something we should force on educators (and let's ignore how you could even enforce the SA part).
I could see a case where a teacher/lecturer might take an OER that was good but fairly generic and adapt it by adding some very local content. This might be specific to the class - e.g. relate to activities they have already completed, or possibly contain information obtained in professional confidence - e.g. patient data, which uses real data from a patient who has given their consent to share some of their details with a given year group. I don't think either of the resulting OERs need to be shared back. The first may not be of any use to others, and may just make it harder to find the original. The second shouldn't be shared as it would undermine confidentiality.
For that reason I think CC BY is as far as we should go, as long as the materials we are re-using and re-mixing allow.