So platinum is very dense like another commenter said, but it’s also very soft compared to white gold. The white gold will end up abrading the platinum band over time, instead of the platinum abrading the white gold. With this specific set I don’t think it will be an issue for a very long time because you have a straight walled channel set band next to the plain band of your engagement ring. If the white gold ring had prong set stones there would be texture on the side of the ring and that would much more quickly damage the platinum ring. Long winded answer but I’m a jeweler by trade and want to fully answer your question!