Dear Vlad,
I play both and it is hard to compare them. Notwithstanding, there are hints.
1. The most important feature is: what piece do you love the most? If you have an answer for this question, you are already done.
2. Look closely at the score and divide each piece into sections. Now, look closely at each section and figure out what will be easy and what will be more difficult. Here, you have many crossroads (hence, a teacher), but basically there is the dispute between what you do best and what you don't, versus if you want a challenge or if you want something smooth. Good questions, but fundamental ones: all your piano playing is about that.
3. If you have time, simply play both. It is not like choosing between two similar works by the same composer. These are dissimilar works by dissimilar composers (and the reason I think your teacher gave you both: to give you a wide range of selection), so the best idea - in my humble opinion - is to study a bit more and tackle Brahms AND Scriabin. The works worth doing that.
Best regards,
Jay.