The Front :
You don't need to flip your phone to unlock it using the fingerprint scanner but the downside is the widely used oval shape making it less accurate.

The Back :
It usually is always on meaning no need to wake the phone first, round and more accurate but you have to pick up your phone each time you wish to use the fingerprint scanner.

So for me that's easily the front, it doesn't make sense to have to pick up your phone to use a fingerprint scanner when it's mostly used as a convenience to unlock our phone faster, so despite being less accurate due to its shape the front one is the way to go imo.

Obviously front fingerprint scanner .. Rear fingerprint scanner usually spoils the looks
But yeah the rear fingerprint has an advantage ..  We can use it to click pics xD

it Looks Cool....

Front one is much more convinient to use. In general it need press and hold to unlock which in my opinion is much more secured way.

Back one is in general always on, which makes it much more prone to accidental touches.

Funny thing about placing fingerprint scanner is its like “wearing underwear backwards ”... And no one wants to do that.. When the design is concerned... If the fingerprint scanner is at back it looks odd ( atleast to me)

i like front finger print scanner. But both has pro's and cons. Like back finger print sensor can also be used for some shortcut gestures like taking selfies.

Fingerprint scanner in the front seems more convenient... Though the design of the button matters too... I like the design of Meizu, Apple and Samsung for their Fingerprint scanner / Home button... I do not like the design of the Fingerprint scanner in the Xiaomi Mi5 and the latest square button in the Moto G4 Plus though... Nexus devices with Fingerprint scanner at the back do not look too bad...

Front side always. But this has a drawback that the screen needs to be waken up before it can be used.

I have Nexus 6p and Meizu pro5.... In the end in term of usage there is not a big difference ( in the beginning i was skeptical about rear FPS)...
But if rear FPS means it has to be always on in order to wake phone... well it means more consumption so i woudl go for front then!