I’ve taken both the ACT and SAT. If she’s good at Science and Math(Stem in general), I think she would do well on the Science section of the ACT. You said she is extremely strong, so I assume she would have no problem with math regardless ACT or SAT. The “writing” is similar for the ACT and SAT. They both focus on grammar, improving passages, etc. The ACT includes a lot more questions than the SAT and tend to include less critical thinking than the SAT. So if you are a fast reader go for the ACT. If you work slower but have strong analytical skills go for the SAT. Lastly is SAT vs ACT reading. The SAT is a lot longer than the ACT reading (65 min vs 40 min). SAT includes more evidence-based questions and you can get away with focusing on only certain sections of the passage. SAT has many questions that often are related to other questions such as line evidence for previous question. ACT does not have line questions, so you have to read the whole passage. I would recommend going with the ACT since your D struggles with the SAT reading. However I would caution that there are much more pressing times when taking the ACT. This is especially true of the reading and science sections