In theory a federal one is better but it's unrealistic. It's going to be hard enough getting a state wealth tax passed, it's essentially impossible to get a federal one passed. For that to happen you'd need to give Democrats complete control of both houses of Congress and the presidency, and even then it would be a struggle. With Republicans in control of anything it's a complete pipe dream. So rather than going for the perfect solution that isn't going to happen anytime soon it's better to go for the imperfect one that has a good chance of actually happening.