It's okay that you're trying to decide to do. I don't think that you're being selfish, and I think that one of the reasons that this is particularly upsetting to you now is that 1) the plating on the gold is wearing off so you can see it better now, and 2) your fiance decided not to get you a platinum ring when it seems that he knew/knows it's what you want and holds up a little better without plating so he could save money, but is now getting himself a platinum wedding band.
Don't make this a huge issue in your marriage, because I don't think he was trying to cheat you out of anything--it seems like he just wanted to propose sooner! See what you think after you get your ring dipped at least once. I think you'll want to do it before the wedding anyway, because there are always pictures of your hands and rings. If you're still unhappy, see if you can work something out with the jeweler, since it sounds like it might be possible to save up for the difference in a couple months if an exchange is allowed. If this is possible, though, be sure to tell your husband that you loved your ring so much that you want to make sure that the prongs don't bend and that it holds up the rest of your lives!
I feel that, especially in today's economy, the best choice is always the affordable one, regardless of yearly income level. I think it is unfortunate when a young couple goes into debt over the rings and/or the wedding dress.
It sounds like you are completely blessed to have a FH who did his absolute best to find a ring that would please you, and that did not break the budget.
That shows responsibility as well as love.
I understand what you are saying about your ring because I am in the same predicament. I personally like the platinum over the gold because it more durable and sleeker and feels better on your finger, but also because I have sensitive skin. The nickle that is mixed with gold to make it white can make some people break out in a rash, especially if the Rhondium(?) plating comes off. One big plus for platinum is the fact that it will always stay white and you wont ever have to keep getting it re plated(i had to get mine re-dipped and it hasn't even been a month yet) So with these reasons and your own in mind, I would say just talk to your fiance about it. I mean its not like you generally dont like the ring itself, its just the metal. I would agree that he might be upset if you didnt like the stone or setting or something but when its just the metal I'm assuming he would understand, especially if you are comfortable with him to go to him with anything(as you should be), he should be understanding, Its your ring, its something youll want to last and have for years and years.So no you are not being selfish, just bring it up casually and take it from there.