The Pixel 6 brought 5 years of software+security updates; the 8 brought 7 years. The 6 started using semi-custom Samsung soc. 8 brought new security features at the hardware level. Beyond that, minor changes afaik. 6 Pro brought true zoom (I really appreciate it), there's a few features that need the 8 soc for "ai", variable refresh rate on the 7 (I think), higher+lower vrr on the 8, etc.

I upgrade regularly: whenever an interesting feature or security improvement is made available, or if a family member needs a new phone (I get the new one, they get my used). Nexus 4, 6, 6P, Pixel 2 XL, 3XL, 5, 6 Pro, 7 Pro, 8 Pro. Toss in the Nexus 7 gen 1 and 2 (cellular) too, and my days of having a conversation with a tablet held to my ear for calls. Family is all pixel 6 or newer right now, and shouldn't need to upgrade for another 2.5y.