The Nexus 6P is kind of odd. Even though the spec says it's the same length as the 6, it actually feels longer and is even less comfortable in my pocket. Maybe because it doesn't taper like the 6, or maybe because it is 1mm longer but rounds down to the same length.
That said, the fingerprint reader is a mixed bag. It certainly doesn't categorically improve security. However, keep in mind that the screen lock and the boot process have the same passphrase. Once you only have to type your password every two days, it means you can pick a very long passphrase with enough entropy that people will be unable to reboot and brute-force your phone's encryption *even* if you've unlocked the bootloader. So that means as long as you don't root your phone (or as long as you have a PIN on root), then it will be hard for an attacker to extract any secrets protected by the operating system (such as your Google Authenticator secrets).
Another advantage of the fingerprint reader is if you *don't* want security. For example, suppose you don't put a screen lock on your phone, but you want to use Android pay. Currently you can't do that. But if you add the fingerprint lock, it's basically like you don't have a lock on your phone, but you can use Android pay and any other features that might require a lock. (E.g., some employers require installation of MDM software that requires screen locks before you can access networks, email, etc.)
The other advantage of the 6p over the 6 is the burst-mode in the camera, which is kind of nice. Also the 6p comes with 128GB, while the 6 tops out at 64GB, and of course the the 6p has a 64-bit processor while the 6 is the last of the 32-bit nexuses. Also, the 6p will support daydream when that comes out, though for now I actually find the 6 fits better in a google cardboard 2.0 viewer.
Having owned the 5, 6, 6p, and 5x, I would say that the 5 was my favorite, being comfortable in the pocket and sporting a super bright screen that's easy to read even in bright sunlight. I'd still be using the 5 except for the fact that it lacks T-mobile band 12 support and WiFi calling. I'm now using the 5x as my primary phone, even though, like the 5, it only has 32GB of storage. I miss the size of the 5, and really don't care about front-facing speakers, so think it's kind of extra stupid that the 5x has a bigger form factor to accommodate what looks like stereo front-facing speakers but is actually a single mono speaker and an identical looking microphone. And while I didn't used to have a screen lock, I now have one since I can bypass it with the fingerprint sensor.