Honestly up until now, those are difficult (also) for me to choose. But let me share my thoughts so that you would find your best conclusion. As LiTTleDRAgo said, you will need Fujisan if you have shortage of awoken burst unleash skill cards (really rare tho). Knowing awoken burst feature is advantage at some competitive battles like EH or even hunting LAWs easily. Another condition is if you want to "get these maiden tickets over with" so badly, then you will pick Fujisan in the end too. On the other hand, you will need Ymir if you focused on 'fast killing machine' card. With her title as most deck cost friendly in realm, yet has abysmal skill makes her the key for ABB whether as attack or defense unit.
Let's getting back to your own choice, hope for its best based from your current circumstances.