Usually only girls get the engagement ring. (when the guy proposes to the girl, he would have a ring ready)
And you wear your wedding bands when you two are officially husband and wife. Obviously, engagement ring and wedding bands are totally different things.
I don't think you have to get a particular kind of material. Platinum of course is a better metal than white gold and yellow gold
ref:
http://en.wikipedia.org/wiki/Platinum
for different style of rings u can see Tiffany's website:
http://www.tiffany.com/International.aspx
I am sure there's Tiffany stores in HK as HK is a big city with great market. I am in Canada and they have a special private diamond section to help u know diamonds better. Go visit! I hope Tiffany in HK is just as good as else where. (I went to the Seattle one and ppl were super nice too.) They won't give you "face colour" if you are just seeing not buying.