Kel did exactly what you're proposing--I'm wearing my promise ring now and we will go engagement ring shopping later. I'll tell you about my ring because the decisions involved some criteria which most people don't consider, but which are important to me.
My ring is black titanium with a tension-set sapphire in the middle. Like this one but with a sapphire, not a diamond:
http://store1.yimg.com/I/titanium-rings_1840_2688141
I dislike diamonds because of the corruption and market manipulation. But, I like shiny things. Also, durability and comfort are the two most important things for me because I tend to destroy anything on my hands. Hence, titanium--strong and light. I also like the tension setting because it doesn't stick out or catch on anything. If I had a stone in a prong setting I'd be terrified I'd accidentally put someone's eye out with it! Not to mention all the times it would snag on my clothes.
I like sapphires because they are a pretty color, rare, and special, very tough--but honestly, Kel thought of that on his own. The black metal I liked because the chemical process of making it black makes the metal 5x harder and more scratch-resistant. plus looks awesome.
But for my actual engagement ring, I was thinking grey titanium and moissonite. Moissonite is more expensive than cubic zirbonium but much, much harder, and without that sort of fake "whiteness" to it. It is also shinier, with more fire, and we've established how I feel about shiny things. But I'm sticking with the tension set, and I've added a new criteria--no little nooks for dirt to get into. I have to clean my ring like every other day because crud gets into the groove--I think it's mostly soap, even though I usually take it off to wash things. Actually I don't think I want to know what that gunk is, I'd rather just have a smooth ring that doesn't GET gunk in the first place! I keep seeing all these fancy, inlaid, ornate rings and thinking they look so pretty, but when I imagine wearing them, I shudder at cleaning it!
Anyway, 2 cents.