I like both. The Magnum 500s look more dressed up in my eye.
Depends what sort of "attitude" you want for the car. Rally wheels look sportier. Magnums look more dressed up, as Tropicalcats said. A wee bit classier, IMO.
You can always have 2 sets of rims. I have Magnums and Keystone Klassics. These are factory 14" with trim rings. I chose these over the Rally rims because I didn't like the '72-'74 "acorn" centercap and wanted a "correct"
wheel for that year so '70-'71 centercaps were out, although I do like them. 15" Magnum rims will fill the wheel well a bit more, don't use trim rings (usually), and have more shine due to the chrome ( I have brushed steel trim rings and spokes).