I also think it depends on what you're shooting.
I'm a newbie, but I do have the 420EX and my wife has a nifty fifty I can use. I find myself wishing I had her 50mm with me more often. I can't use the flash everywhere (not allowed/appropriate, or the space is too large), so the 50mm would be better.
However, I'm surprised at what I'm able to do with the flash - without really knowing how to use it. This is mainly when I'm shooting inside the house with my 60mm macro, but it will work with your current lenses too.
I just set the camera to manual, shutter speed to 1/250, ISO to 100, and set the aperture at whatever I want from 2.8 to 5.6 I think, and I get pretty good exposures I think. Setting the aperture much smaller than 5.6 begins to give underexposed shots.
As I said, I don't know what I'm doing with the flash - but what I say above works well enough for me right now.
However I'll say that my sister's wedding is this weekend. The wedding is outside, and the reception is in a barn. I haven't seen the inside of the barn, but I expect bad lighting, and a high (and probably brown) ceiling. This all means that I probably won't get much use out of my flash (though I'll try). But, I'm sure the nifty fifty will come in handy. But, my wife will probably be using it.
50D - Canon 60mm Macro - Sigma 30mm - Canon 70-300mm DO IS - Canon 17-85mm IS - Canon 50mm 1.8 II