And white gold vs platinum, well tehy are just to different beasts. Intitially you won't see any differences at all as the white gold is rhodium plated. Where you will see differences is down the line. Platinum develops a patina...kind of a matte look. White gold stays shinier but as the rhodium wears off, the metal is less "white". There are many levels of white gold. Some show a tinge of yellow after a while. White gold can be rhodium plated every years or two to restore the shiney white look. Platinum can be polished to renew it's shiney look.
It's really personal preference. I like white gold better because it stays shiney. Other people like the patina of worn platinum, If your solitaire is platinum, I would stick with platinum so that they match over time.
Also people who are allergic to nickle can have trouble with white gold, or can develop an allergy over time. Most people don't, but it can happen.