Parasite said:
If Colony trading is forbidden for some reason, ice is still just as good as rock. There is a minor issue I have heard of in that the methane ice worlds have one less picture, so are a bit less common, but this is hard to test without creating multiple maps and doing counts with error bars ect.
If you count the entries in SectType.txt, it is readily apparent that Methane has one of its entries set to the wrong type. Alternatively, you can use SJ's Map Analyzer to do the counting for you.

One of the images that should have been used for Small Ice was instead used for Small Rock. This is the source of the imbalance, thought it is fairly minor in the big picture. If you use the Map Analyzer, you can see what the exact effects of the imbalance are.