My first digital in 2003 was a 3 mp and gave excellent 6x4 prints and pretty good A4 size. With a higher mp you will be able to crop photos down to a smaller area and it will also give a sharper picture on your TV if you display that way. A 12mp should enlarge to poster size with little trouble. Provided the camera is set to the highest resolution that is. 12 mp cameras are so cheap now that I wouldn't hesitate. I think the cropping results would be where you would benefit most in normal day to day use.
What is Amazon Go and will it come to the UK? The store without checkouts or queues