Are you choosing between the two? Or are you upgrading from Pixel 1?
Well first, the Pixel 2 removed the 3.5mm headphone jack. As far as their aesthetics goes, they almost look alike. Pixel 2 has added a feature of squeezing the phone and acts as an app launcher. It's got 2700 mAh of battery and it's a little smaller than the Pixel 1. Pixel 2 also has a much faster CPU.
If you are choosing between the two, I think you should go for the Pixel 2 because it has better specs than the previous model and the Android updates will be supported for three more years. But if you already have the Pixel 1, I'd rather not choose to upgrade because most of the improvements are specs bumps and the Pixel 1 can also do what the Pixel 2 can, only a little slower.