I tend to think Unitedite is more serious and proper, whereas Blade is more "one of the lads". Sign of the times, I suppose, that no one wants to be serious and proper, however it always sounds wrong when someone uses Blades in serious conversation. I don't particularly use Unitedite, but if I'm talking to non-Blades I don't use Blades, I use Sheffield United the first couple of times, to make the point that you can't say just Sheffield, then I shift to using just United to make the point that it isn't the name of ManUre. I know, very anal. If I refer to the fans I say United fans, I should probably change to Unitedites.
A couple of things I don't like - first is the way the club have started calling themselves the Blades; I do think it's more a fans' word and, as I say, doesn't sound serious enough for the club to use. The other thing, possibly related, is the term 'Blades fan' which has crept in. It's Blade, unless you're talking about your own fans, of course.