The O2 had a built-in keyboard which allowed for general text inputting (e.g., high scores), and the eventual purchase of a cheap "learn programming" cartridge for me. I was coding in assembly and machine language before I was a teenager. It is probably the reason I am a software developer today.
The O2 had well-made joysticks; my original, ~40-year old joysticks still work like new today. The Atari's sticks were poorly designed with that plastic flex-return mechanism and short-lived microswitches. I had to fix so many of those over the years for friends.
The Atari usually had better graphics, but the O2's screens were always flicker-free; something the Atari could not boast for every game.
Atari's library of games was huge, but some of them were poor. O2's line-up was thinner, but some of the games were quite creative, complete with game boards, tokens, and playing pieces. (My Minecraft-playing niece and nephew say they love "Take The Money And Run!") It was a lot of fun, and is surprisingly still entertaining.
Either way, I know Ralph Baer's story, and I respect what he's done for (to?) all of us. I thank him for creating such a clever invention.