Hoot has the right idea. Copper is better at heat tranfer and has a good thermal capacity, but aluminum is not far behind in either regard and you will probably not see much of a difference between the two different metals. Minimizing the battery effect will reduce the possibility of complications in 6 months.
Run deionized or distilled water with waterwetter in a single metal system or run 10-20% antifreeze, waterwetter and DI/distilled water in a mixed metal system.