I like the round one better. BUT, you already have a round ring. I'd personally go for the princess ring to have something different. If you're in the mood to wear a round stone, you can always just pull out your old ring and wear that. Kwim?
WOW! Both stunning rings. Congrats!
Out of the 2, I love the way the round cut looks on you, it looks so elegant. Like crys mentioned, I also like the way it matches your wedding band.