PyTorch is almost (or even literally?) a fork of chainer, which can be seen when comparing example code. The latter was much more stable than the former for quite some time after PyTorch gained big popularity through Facebook. We have been using chainer for a lot of published NN research projects and only recently moved to PyTorch because students complained that they feel they can't put the more popular framework on their CVs..

I continue to have more sympathies for chainer.