Where would I put GRUB for a dual boot?

It depends which bootloader you want to use. Personally, I use Grub, specifically Burg - this is what it looks like:



I have Mint and Windows on my SSD (dev/sda) - which has Grub in that MBR.

Recently I messed with the partitions on my second HDD (dev/sdb) and lost Windows boot - Grub's osprober found windows OK but windows still would not boot
Macruim Reflect's "fix Windows boot" only managed to overwrite Grub, but the boot error persisted.
The way I fixed it was re-installing Grub from Live, then running diskpart from the Win install media to get the system and windows drive letters - ran chkdsk /f /r on both those - came out clean - what fixed it was bcdboot [windows drive letter]:/windows.

Just relaying my experience as something to bear in mind when setting yours up.

Superfly said:

It depends which bootloader you want to use.

And which OS you consider primary, I suppose.

I took the non-conventional approach.
This is on a MBR / BIOS machine
- Win8.1 primary, Win10 as 2nd OS, Linux Mint as 3rd OS

I GParted two separate partitions (ext4 & linux-swap) before installing Linux Mint
I had to tell the installation to use the ext4 part (it was flagged "do not use") - set it to use ext4 journalling
I selected the ext4 part for the install and set the mount point as / on that part
(be careful - the install seems to want to use to sda. just change it to your ext4 sda ... sda6 in your pic)

edit+
Make sure to remove the the Boot flag from the ext4 part if present
I used unallocated space in the extended partition for both Linux parts (as logical parts)
edit-

I don't see a linux-swap part in your pic.
I'm not certain, but suggest that you create a small part in GParted with a type linux-swap
I used 1024 and it seems fine

Then you have to tell Bootmgr about Grub- I used EasyBCD
Pick the Linux OS tab
Point to the ext4 part

The advantage?
Each OS leaves the other boot alone - sure you might have to reconfigure Bootmgr to know about Grub after an update from MS, but you should always be able to boot to Windows.

edit+
Superfly's post below led me to this: Dual boot Windows 7 and Linux/UNIX (also MBR / BIOS information.

Unix BSD descendants need a primary partition. Linux descendants can be installed to logical partitions.

edit-

Thanx Bill,

The other thing is... for adding Linux boot to the Windows Bootmgr in UEFI, try BootNext tool - boot any Linux or Windows from Windows 8.1/7/Vista - I don't think there is a need for legacy mode.

Thank you Superfly,

I wasn't sure about UEFI ... I added an MBR disclaimer in my post.

Superfly said:

I don't think there is a need for legacy mode.

You're talking about legacy mode for Linux, yes?
I agree.