I cant find any reviews for the SF540 steamer? Not listed on amazon either? Is this a gamble not having a physical store nearby to return it to if its horrible?
Well, it has a larger tank than the SF-510, and I would assume otherwise it is like the SF-510 but made with slightly higher quality components and a more professional look. It at least comes with a crease tool, has a nicer hose, and can be hooked up for "continuous fill".
Of course, that the higher-end model is better could be completely wrong in a few situations, but it's probably safe to assume it's of better quality than the SF-510.