Platinum is better, if you like your jewelry white.
the only question is, can you afford the platinum? There is a considerable price difference, though its not as bad as it was last year.
A good quality white gold is fine for earrings and pendants, but rings get a lot of wear. And as people have mentioned, they buy white gold because they want it to be WHITE. But gold is so yellow naturally, that even with all the alloy in it that makes it white, it's going to have a yellow tinge. So jeweller's solve this with rhodium dipping, which doesn't last (in a ring)and needs re-dipping and is a real pain. And you have to avoid swimming pools and take more care in cleaning (can't use ammonia based anything) and its not as convenient at all.
Platinum does get the patina, but many people like that. I don't know if I would or not, but I use a jeweler's rouge cloth on my platinum rings every once in a while and so far they haven't gotten any patina.
Older white gold is a different story. My grandmother had a vintage 20's ring in white gold and it was not rhodium dipped and did not ever get yellow. But I think since after the 1950's or so, the rhodium thing has been going on and while there are always 2 or 3 people in a discussion who have great body chemisty (their perspiration isn't as acidic or something) who will have had a rhodium dipped white gold ring that has never changed, it's jsut not common to be like that. So I'd wear gold for a ring only if it were yellow gold, and buy platinum if I wanted something white (unless, as stated before, it's earrings or a pendant).
It would be in jeweler's interests to have better disclosure about this subject, IMO.