The content for both tests is much more similar than it used to be (the SAT made a number of changes beginning in 2016 test that make it much more like the ACT). The main difference between the SAT and the ACT now is that the ACT – as you noted – has a science section (which is not tied to curricular content as much as it tests your data analysis and reading comprehension for facts and analysis) and the SAT does not. There are also differences in the timing and execution of the subsections (which, before the SAT redesign, were very different); the timing for the two tests is much more similar now.
Were the problems you encountered with the science section more a matter of running out of time or did you have difficulty with the questions? The timing issues are easily solved through practice and developing good test-taking strategies (e.g., look at the number of passages/graphs that you will need to tackle and make sure you have enough time for each; don’t get hung up on any single question).
There used to be a poster on CC named Ziggi that had a lot of very useful advice regarding test prep and helped many, many people improve their scores. While some of it may be dated (it was before the re-vamped SAT and before Khan Academy had partnered with the College Board), most of what Ziggi suggests is still relevant. There is a 73-page thread discussing his methods; one of the posts (1022 on page 69) links to a quick overview. Here’s a link to the thread
http://talk.qa.collegeconfidential.com/sat-preparation/68210-xiggis-sat-prep-advice-p69.html
Ziggi’s method involves using ONLY released tests, working through them one section at a time, and looking at solutions for ALL of the problems – not just the ones that you got wrong. In order to understand the strategies you need to figure why the correct answers are indeed correct (just because you got an answer right doesn’t mean you know the best way to approach a given problem). It’s important to understand the patterns of what the testmakers are looking for, the formats in which they typically present certain problems, and how they can be most easily solved.
There are a few tricks and shortcuts that you can use once you recognize a specific “type” of problem. A good example of this is math is how to approach the ubiquitous average speed round trip travel questions. For the English writing/usage problems, there are certain things that the tests test (e.g., subject/verb agreement; possessives: its/it is; their/there/they’re; punctuation; finding the simplest and most direct way of saying something).
If you indeed are able to spend and hour per day for the next several months, you should be able to see a dramatic improvement. Once you have become comfortable with how to approach individual sections, take a few full-length practice tests under simulated testing conditions.
Given that you did similarly on both tests, you might want to focus on the one you are most comfortable with, but the content is similar, so improving on one will help you improve on the other.
Good luck!