I prefer the platinum, but I think that you should go with the one you like best. If there's a clear winner in your mind, don't let anyone talk you out of it just because it isn't classic enough. On the other hand, if there isn't a clear winner, then I think you should go back and try them on again, spend some time thinking about it, and wait until you're sure before you make any decision. You want it to be a piece you will love forever.
Personally, I love the Tiffany Victoria in the platinum – but then, I prefer white metals in general. With that said, I do like the aesthetic of a diamond pendant on a gold chain, especially when the shade of gold is somewhat similar to the wearer's skin tone. My own favourite necklace is a DBTY solitaire in yellow gold, and I love the way that the diamond pops while the gold chain recedes into my skin tone (my skin tone is actually far more pink than yellow, but I'm just not a fan of rose gold). On the other hand, I still prefer the Tiffany Victoria in platinum, and to me that will always be the ideal pairing.
I gather from your user name that you like pink a lot more than I do! If pink is your favourite colour, then I would imagine that preference won't change even if trends do. It might be useful to think about yourself – rose gold might be the trendy metal of the moment, but if you have an enduring love for pink then will you care if the trends change? That might be the most important question to answer.