kubuntu - On which device do I install the bootloader when I want to dual boot?

I'm currently trying to install Kubuntu 15.04 alongside Windows 8.1 (already installed).
It's on a non-UEFI machine.

My partitions are as follows:

• sda1 windows installation (500GB SSD)


• sdb1 HDD just for data (1.9TB)


• sdb2 very small partition that was already there don't know what it
  does (367MB)


• sdb3 the partition I created for Kubuntu (100GB)

Now in the installation menu of Kubuntu where I do the partitioning and so on I also have to choose which device the boot loader gets installed on. Which one do I choose for that?

sda1 where my windows installation is or sdb3 where my Kubuntu installation will be?

The bootloader is for choosing which OS I want to boot in later right? But I have no idea where I have to install it.

Edit: This is what it looks like