I did the ACT with writing in December and got 30 composite and 33 in writing. I did the ACT again in April and got 32 composite but my writing dropped to 25. Should I send both or just one? If so, which one should I send?
It will depend on the schools you're planning on applying to. If the schools you're applying to require all scores sent, then it's a no-brainer - you have to send both sets of scores.
The next factor is whether or not the colleges you're applying to superscore the ACT. If the colleges you're applying to take your top score from each section and average those together, then it's definitely in your best interest to send both your scores. If, on the other hand, a school explicitly states that it only looks at your highest composite score (like Vanderbilt), then you should only send your 32 composite score test.
If the schools you're applying to don't require all scores sent and don't superscore the ACT, then there are a few things you could do. There have been some issues with ACT Writing scoring, so if you're concerned that your April ACT essay may have been scored incorrectly, you should definitely think about getting hand scoring (they'll only refund you your money if they made an error, but they'll also only change your score if you do better on the rescore - otherwise, you get to keep your old essay score).
There's also the option of taking the ACT one more time WITHOUT Writing in an effort to boost your composite score. If you get higher than a 30 composite score on that test taking, then you can just send in your 30 composite/33 Writing and new 30+ composite scores to schools - you'll get the benefit of a higher composite score as well as the benefit of having scored a 99th percentile score on the Writing test, and the 32 composite/25 Writing test doesn't have to be an issue.
If you're not planning on re-taking the ACT, however, then you should submit both scores. A 25 on ACT Writing is still a 90th percentile score, and a 32 vs a 30 composite score could make a big difference if you're on the edge (e.g. if your GPA is lower than the average student accepted to the schools you're applying to).