Keep in mind that the ring isn't worth much money.
I got the 0.71ct diamond (GIA certified) and the ring from a wholeseller in Asia. The diamond is worth around 3k Canadian dollars while the 18k white gold ring (any design) is only ~120-140 Canadian dollars.
If I get this combo from a retailer instead of a wholeseller, it would cost around ~5k Canadian dollars. The markup is totally ridiculous...
Regarding style, you can pick something classic so it's timeless.. or you can pick something more special. Few years down the road, you two can always pick another ring and fit the same diamond in... as the ring is relatively worthless compared to the diamond and it's the diamond that is symbolic.
So, ask yourself how much you are willing to spend and the 4C - Color, Clarity, Cut, and Carat.
I would suggest you to get a diamond with colorless grade F (cheaper than D and E while still being colorless, or it would look yellow when the bride is in her wedding dress), Very good/Excellent Cut, Polish and Symmetry, no Fluorescence, and SI (slightly include) or VS (very slightly include) Clarity.
SI1 and SI2 actually don't look bad depending on where the include is, an amatuer may not even be able to spot the flaw with 10x magnification while you could easily spot the flaw in VS1 and VS2 clarity diamonds, again, depending on where the include is. However, the price difference between VVS1, VS1 and SI2 could be quite big (assuming the Cut, Color and Carat are the same). Also, price jumps dramatically when you get over .80 carat (while the size doesn't appear that much different).
The important thing is the GIA certification, and make sure you match the serial number inscribed on the diamond to the serial number on the certification 1) when you first bought the diamond and 2) when you take delivery of the diamond ring (the jewellers should know better than to hide the inscribed serial number with the prongs of the ring that hold the diamond). If you don't know much about diamonds and you don't plan to get a GIA certified diamond, you may as well throw your money away.
It's easy for the jeweller to adjust the size of a white gold ring... can't say the same for platinum. http://guide.diamondpriceguru.com/res/img/gia_1.jpg
Make sure you do more research before you head to a store so you know what to expect.