I would choose A. This is about him buying you an engagement ring, and even though it would not be the one you want, you could always upgrade later. But, I think that he needs to pay for it.
I was married once before, and when my ex-husband (now friend!) proposed, it was a simple 1/3 carat solitaire. And it felt right. We did upgrade later, he bought me a 1/2 carat diamond, and I bought the platinum setting. When we parted ways, I gave him the stone back and I kept the setting.
Here is what my SO and I are planning on doing, if it provides any insight: last year, we were at Cartier, and he mentioned how cool it would be to have LOVE bracelets instead of wedding rings. I thought the idea was so awesome.
Our six year anniversary is this August, so we talked about the LOVE bracelet idea, and we are going to give each other bracelets as engagement gifts. This works out really well for us, because he will be able to buy one for me, without using credit (which I think in this day and age of credit cards is wonderful), and because we have always split everything down the middle, this will be an even exchange and we both get something for the engagement that we really want!
I hope that helps!