I've used Fimo (oven-bake polymer clay) and Apoxie Sculpt and I like both, I think it depends a bit on what you are doing with it.
Apoxie is definitely the most durable option so it's my #1 choice for any long and/or thin parts like horns, dresses, vines/swirls... I actually do most of my sculpting with Apoxie these days. It can be rolled so thin it's almost see-through and it still remains quite durable. I really love that you can work on it even after it's cured, it takes sanding so well and you can take a knife to it. An X-acto blade is my favourite tool to sculpt with, both before and after the clay has cured! Fimo, however, is almost impossible to sand or cut after it's cured, but since it has no limited working time like Apoxie, you can usually just smooth it before you bake it. It's much harder to blend seamlessly than Apoxie, though, so it's not as good for fixing bite marks, reposing, sculpting feathering etc. Those probably require some sanding afterwards, no matter how well you smooth out your clay.
I would use Fimo or other polymer clay rather than Apoxie for any parts that require a longer working time, however. I still find Apoxie harder to use for any detail work... but I guess I just need more practice. Fimo is also more flexible than Apoxie, so even though it breaks more easily, it's great for e.g. masks that need to fit tightly but also come off easily.
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loginFor Renji's sword I used both clays. The sword itself was made with Apoxie, there's no way I would've been able to do that blade with Fimo. I rolled the clay thin and only after it was cured, I cut it into shape with an X-acto. The guard and other details were added after that. The scabbard was made with Fimo as I had no success with Apoxie, it adheres a bit too well sometimes!
I wrapped the blade in cling film, covered it in Fimo, moulded into shape. Then I pulled out the blade, removed the film as it would melt in the oven, put the blade back in and baked the whole thing.
I think every type of clay has its pros and cons. It may take some time to experiment with different types and find your favourite.
Careful packaging is also important! Any clay can break if handled too roughly. Good luck!