Having used both iPhones and Nexus phones with fingerprint sensors the back position is much better for me. My index finger rests there naturally when holding the phone while the home button placement is awkward.

But this way you can't unlock it if it's sitting on a desk or whatever face-up. That is where the finger rests during use, but you usually only want to use the fingerprint reader before it is resting there, to unlock the phone when you begin using it. Most of the time when you're using the device you're not using the fingerprint reader.

I've found I can also maintain a better grip on the phone (3 fingers + palm/thumb) while pressing it over the home button (sort of cradling it in four fingers).

I have massive hands though so it might not be an issue for most.

I have comparatively small hands. On an iPhone 6, I usually unlock it with most of my hand near the bottom of the phone to get my thumb in position there. After it's unlocked, I shift my hand so that my thumb's coming from the side of the screen, instead of the bottom.

In that latter "using the phone" position, my forefinger rests on the upper-middle of the phone, right where the back-side fingerprint readers are located on many (most?) phones. I feel like a reader in that position would make it easier for me to pick up the phone and unlock it without needing to shift my hands back and forth.

All the Nexus uses I know prefer it there to the home button location like the iPhone fwiw.

On the back is definitely better.