It depends on the type of lenses that you have or plan on getting in the near future. The D60 will not provide AF operation on non AF-S Nikkors or other brands with out a focusing motor built into the lens. Your D50 will. The D60 is a little smaller and lighter than the D50 and has a few more MP. Good Luck and Enjoy your Nikons!
As far as the megapixels are concerned, I don't think there's a big advantage for most uses. I have a 6mp D50 and a 12mp D300, and have printed as large as 20"x30" with both. It's not obvious, looking at the prints, which ones were made by which camera. The D60 will allow you to crop more, and I won't say there's no advantage to the additional megapixels; just that for most purposes I don't think the difference would be visible. Do you notice a lack of sharpness or detail in your D50 photos? I don't.
As for smaller and lighter, I do see that as an advantage. Often I'll carry my D50 with me instead of my D300 because it's smaller and lighter (and in most situations, the results look the same). A D60 would take that advantage one step further.
If I were in your situation, I'd just stick with the D50, but you might weigh the advantages of the D60 differently, and I wouldn't argue with you.
D60 as a 2nd body isn't a bad deal, but I wouldn't use it to replace a D50. You do get smaller and lighter with D60, and a better rear display, but you lose AF & AF-D compatibility, and auto exposure bracketing, which are things I wouldn't live without.
I bought a D60 as a spare for my D90, because the D60+18-55mm vr kit lense is a very light weight combo(climbing the Alps). I was very suprised with the great IQ the D60+18-55mm vr setup offered. Stick with Nikkor lenses like the 18-55mm vr and 55-200mm vr. It will save U $$$ and you'll have great IQ.