Well you can only really go with whatever your feelings tell you. It seems like you have a good thing going already. But if you love the new girl more, then who knows.

Although it's hard to really tell much from the limited info you've given. Like if you could define nice maybe?

But anyways my best advice is think carefully about it, in the end it's your choice, so you need to really think which one you see yourself with and truly feel right with.

Well hope that mild advice was slightly helpful in your thought process.