I've heard that the V60 is better for light roasts and the Aeropress is better for Medium to Dark roasts, but could someone describe how these two brewing devices affect the flavor of coffee brewed for the different roasts (and which, in turn, would be preferable)?
That sounds like the generalization that drip or pour over brewing is better for light roasts and stronger brewing methods are better for darker roasts.
I'll counter that by saying that I often use my Aeropress for lighter roasts and my Clever Dripper for dark roasts and those cups come out just fine.
The Aeropress makes a sort of quasi-espresso cup that isn't true espresso, but is much stronger than just strong coffee. I've found that it is unsuitable or less than satisfactory for beans that aren't recommended for espresso brewing. While the Aeropress allows less control over "brew" strength. The V60 should be more flexible. Varying the grind and pour speed should allow a level of control not present with an Aeropress.