I can somewhat relate to how your dilemma is. I'm not sure what exactly is giving you the hold up, but a bit on my story.
I initially purchased the plain yg love because I couldn't quite fathom paying much more on the 4 diamond love. I even debated on 2 plain loves to stack. My heart always ached for the extra sparkle even though I didn't want to pay that much, as time went on, dh saw that I wasn't really in love with it and didn't wear it much like I'd wanted to so he decided to upgrade me to 4 diamond. It worked out to where I actually gave my plain love to my mother, so I didn't feel too sad to let it go.
Maybe it's the fact that you get to have 2 different pieces to enjoy vs one that's giving you a hard time. However, diamonds for me ***** plain just because I love diamonds and the 4 diamond (which I have) is such a beautiful piece and just gives it extra specialness. I always second guess myself and dh has always told me, follow your heart, if the diamond one is what you really want, you should go for it because in the long run, you'll be wasting more money having to re-purchase it.