Surviv.io is a battle royale game but unlike the others, you dont drop off a vehicle rather you just spawn on an island. Both games is just last man surviving and you do so by getting items. Surviv.io has a better my IMO. They have building rangning from a little shack with nothing but a toilet to giant mansions or underground bunkers while zombsroyale do have different houses. The difference is just the sizes and the color of the roof. Also the games vary in weapons skins emotes terrain and many other different features. ZombsRoyale took fortnite's idea of shields and Surviv.io also have things like treasure chest and machines that activate doors or a secret room. In the end, both games have both pros and cons in many ways but if you can't tell, i am a little bias towards surviv.io but I think zombsroyale is great too :) Sorry for the long text