Use both is the simplest answer for a software developer.
If you want to excel at being a cracking engineer you need to understand both as you may end up working with Linux and Windows in the same environment.
The advantage of Linux is that its open and you can learn the inner workings of the system and browse the code easily which is a massive plus for system engineers as changes can be made on the fly.
But Windows is also massive in enterprise and learning Windows operating system is a massive plus to you, what if your code is running but its causing faults? understanding things like sysinternals can be helpful to you.
Just use what you fancy at the time or run one in a VM and enjoy it.