My husband and I got two identical bands for engagement rings.
They were cheap and made of titanium - "our rings will last as long as our love; d'awww!"
We had a "long" engagement, though. Over two years. I wouldn't bother if it's short. We bought prettier bands (not identical) for the wedding.
I could never have a ring with a "rock" on it. I'm too clumsy. I'd catch it on everything. Also, don't ever buy diamonds. Not because of the blood diamond thing, though that is bad, but because De Beers has a bit of a... monopoly on the diamond market. If you mention that to a salesperson, they'll say it's good because that means it's quality controlled.
Bullshit. Diamonds aren't rare. Yes. They are pretty. And if your girl moans about them, get her one. But their price is HIGHLY inflated. Rubies and sapphires are nice hard stones, and long wearing. And they are beautiful. Sapphires come in tons of colors. They are both rarer than diamonds. So are emeralds, but you don't want an emerald on a daily ring. Too soft. It'll scratch right up.
From what I understand, you can get a "fake" diamond (though it is chemically identical) for a portion of the price. There is no physical way to tell the difference. Plus, they can give you all the rare colors. I'd say look into that if you've got a diamond girl on your hands.