Rim type depends on a lot of different things as Oliver touched on above. Bob, definitely go check out the Comet Kart site as I do recall Mark Dismore posting a very informative piece on the differences between magnesium and aluminum.
Just a couple additional points from my point of view: Since magnesium does not conduct heat as quickly or as easily as aluminum, it generally allows the kart to stay freer for a bit longer. All karts tighten up during a run, regardless of what you do. The key is to keep the kart as free as possible for as long as possible. Particularly on really grippy surfaces, magnesium wheels are a little better at holding off the heat transfer/build-up than aluminum wheels. I also think this is more important with the 7.1 tires than the 6.0 tires. But regardless, anything that will help to keep the tires cooler, will help the kart’s handling.
By the same token, we run aluminum wheels with our rain setup. There, we want the heat to build up to help enhance what little grip we can get.
As always, there are way too many variables to just say you want magnesium wheels in X situation and aluminum wheels in Y situation. Track surface plays a part, kart brand, weather, set-up preferences, etc., all come into play. And I would also say in some cases, the differences are so minute that some drivers may not be able to tell the difference or even care.