I'd say it depends on the type of game you're making, if you're making a simple game use 8-bit as it fits your theme but if you're making a more fully fleshed out game then I'd suggest 16-bit. Personally I prefer 16-bit as it's more visually appealing but harder to make than 8-bit. Good luck with your game

Also could you give us a link to the game 'Tiny Guns', as I've never heard of it before?
Edit: Okay, I've found it, I would say that's 16-bit.