its about the 5th time i read that question during the last days, and i have no idea WHY ppl even ask it. the two games could not have less in common.
They are both, at their core, about expanding a base surrounded by groups of hostiles that occasionally attack and making sure that your "economy" is well macroed and expanded at a rapid enough pace. I can certainly see the similarities.
That said, they are still quite different games. They are billions is... I think, important to be hard, as there's not a lot of game there otherwise, it's mostly just trying to survive, therefor that becomes the focus of it, surviving and the difficulty of doing so, so you'd better play on more or less the highest difficulty you think you can win on, because that's what makes it fun, and that's all the fun to be had, in desperately macroing up your economy. Factorio has a lot more depth to it, so, for me at least, I don't need a challenging game to have fun in it. In factorio I can fuck around forever with building a factory and production lines, because that's fun in of itself (unlike placing an woodcutters hut in They are billions at least). That said with the right settings, I think you can probably also have a challenging game in factorio, but it's less important, and the game is probably less tuned to it. So basically what Retropunch said, they are billions is fantastic for a defense game, factorio wins everywhere else.
For my money Factorio is easily better if you have friends to play it with. Mutliplayer factorio is a real blast to play (I know any multiplayer game is more fun then a single player game, but factorio has a lot of the right stuff to make it really shine in multiplayer). I think they are billion is a lot closer as a single player experience, but that's me personally, if I'm alone I can only fuck around with circuits for so long, I think, so I think I'd say I like they are billions better single player? Maybe It's hard to judge, as thinking about it I've never really played either game alone, I've only played factorio multiplayer and played they are billions when streaming, so I guess I don't really know if I'd be up for they are billions alone, but it feels like I could play it alone.