i do just the opposite, white gold engagement ring with platinum wedding band. the only con that i think of is that the two metals "age" differently... the white gold turns yellowish over time and the platinum stays white but loses its shine. i know i must have the terminology all wrong, but hope this helps
after buying my ring I don''t think I can afford a matching platinum band yet .
I lost a ring in the few years ago because of wrong sizing and so I was thinking if i can just get a white gold band for now to go in front of the ring (kinda protecting it from falling). I am def. getting the right size this time but that fear of losing has stayed with me