As far as I'm concerned, there are only two attribues that matter here: focal length and maximum aperture. Whether one lens is "better" than another in any other respect is probably less relevant in handheld night photography than anywhere else, because:
- subtleties of contrast are lost at night - there's bright and dark and little in between, and any lens can deal with that
- subtleties of color are similarly lost at night - there's orange incandescent light and brightly colored neon light and little else
- the lens is not going to be your biggest limiter to sharpness - noise from high ISO and shake from slow shutter speeds will be
Sure, it's *possible* to come up with counterexamples to these observatin, but I'm talking generally. As long as you lens has the focal length(s) you want and is reasonably fast, it will work as well as any other.
So I concur with the general idea that the 50 is great simly because it is the fastest, and the 18-55 is nice because at the wide end it just might be fast enough. Between SR and the possibility of resting against posts and so forth, you can shoot handheld at relatively slow shutter speeds and get away with it.
If I were taking just one lens for this myself, it would be my M28/2.8, which can easily be had for $50 or so. Here's a shot from it on my K200D at ISO 400, f/2.8, shutter speed 1/6", which was made steadier by leaning on a railing:
The 18-55 would have been at f/4 or f/4.5 at this focal length, which would have meant upping the ISO more, which would have been OK too.