It really is a catch 22.
Buy new, buy potential problems, buy at the brand new price, but buy the most security service life you can get.
Buy a year old, buy a more proven design, buy at a reduced price, lose a year of security service life.
Google's policy for Nexus and Pixel devices:
- Android version updates for at least 2 years from when the device first became available on the Google Store. After 2 years, we can't guarantee additional updates.
- Security updates for at least 3 years from when the device first became available on the Google Store, or at least 18 months from when the Google Store last sold the device, whichever is longer. After that, we can't guarantee additional updates.
Apple doesn't seem to have a published policy, however, they do seem to try to get the latest OS on to the oldest device possible, even if it means disabling a few features on the older devices. For example, the iPhone 4S was supported until iOS 10 was released last year, so nearly 5 years of OS updates.
I'm currently on a 6-8 month trail of an iPhone 6 that was my wife's as we just got her a 7+. I'm trialing iOS on a daily phone basis, the smaller screen (as compared to my former Nexus 6) and waiting for the next iPhone. I still probably will not get it within a month of release, so maybe I'll get it in December as they are normally released in November.