Jim,
Stan's suggestion is a good one. If you discover that the apparent differences in effort are, in-fact, due to the boats and not you, it won't likely have much to do with the hard vs. soft chine question. There are lots of other variables in hull shape would probably have more to do with the discrepancies. If for example, your boat has a higher prismatic coefficient (fuller ends below the waterline) than the others, it could be more efficient at higher speeds and possibly easier paddle upstream against the current where your effective speed through the water might be higher. When you are more tired and now going downstream, your speed through the water is probably slower and now the boats with soft chines and lower prismatic coefficients (less drag and easier to paddle at lower speeds) will have an advantage. That's a total guess, but it might explain any actual upstream/downstream differences in boats you might discover.