Lots of good wheel discussion here: Are MXC wheels worth the money?
Otherwise, as Dom said, aluminum wheels will heat the tire up more and are cheaper to replace in the event of damage, so they are the preferred material for rain conditions.
All other dry conditions, magnesium wheels are likely to be a better choice. Wider or narrower wheels with the same width tire will change the amount of sidewall flex in the tire during cornering. This can be positive or negative depending on what you’re trying to achieve.