As has been said, Smarty is owned by the same company as Three and use the same masts. (So network coverage is identical).
The only real difference (apart from pricing) is that, while you can call a support line to get help with Three's services, you have to go online to get support with Smarty. (That support is initially via an automated bot but between 0800 and 2000 you can get referred to a human operator if need be. You still need to type your side of the conversation though; you can't speak to them directly. Support is also available by email).