I'm feeling overly opinionated this morning so here it goes..
I'm more of a traditional guy. I will go out and get a ring for you and if the whole love thing is the most important aspect of it then it shouldn't matter what the size or shape of the diamond is. I've seen women tell their guys that if you get me a ring it better be at least one carat......Yikes. and in all honesty I'm pretty sure most women agree "I never met a Diamond I didn't like"
I do believe you should be down on 1 knee w/ ring in hand and do it proper, Special place special time.
Bottom line is you Know her and know what she likes. You do what will guarantee your best chance of survival in the coming years.
all kidding aside marriage is a great thing. I've been married 12 years and have a wonderful wife and 2 great kids.....enjoy