The obvious advantage to using prime lenses are weight and speed (compared to zooms) and therefore i recommend that you consider those factors when making your choice. If the lens is not significantly smaller or faster than what you have now, it is not worth the bother. All that being said, I say go for the 24 - f/2.8 or faster. 35 and 28 are much too close to the 50 for my taste. And everyone has to have a fast fifty, right?