Deciding which ring you like is a very important aspect of the purchase. With the engagement ring you give her, you are expressing yourself. Every time she looks down at that ring (which she will every day), she will think of you. What do you want her to see? Your friends and family will oooohh and ahhh over the ring and they will compliment her, but they will know that you picked it out. Choose one that you like so that you can stand behind it with pride.
If you don’t have strong feelings about one or the other, do some research on this website about the different meanings of stones, different cuts and gradings. Then start looking in shop windows and see if you form opinions. You might notice there are some rings you don’t like and that can be a starting point for developing your own taste.