A block with a better thermal coefficient will always be better no matter what the dT across the block is (if all other factors are equal).
Where things might change is if you are dealing with factors that are not consistent across temperature such as viscuosity. This is a very real consideration when dealing with very low temperatures, as you will need to use coolants other than pure water. When adding glycol-type antifreezes or methanol or using non-water coolants, you may move to a design that is less sensitive to viscousity.
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