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This is something I can spend hours fretting about...so I know how you feel. I like to consider how it looks from the back...perhaps that is why it makes it a difficult decision? Using the same colour as the hearts would look fantastic...but I guess it depends on the look you are after. Sorry I am not much use here....but will be following the thread with interest.
By the way....it's a beauty. It's going to look fantastic quilted. Please keep us posted on what you decide.
I'm going to stick my little wrinkled neck out and say that I would use a color in a slightly lighter shade of that heart's color to quilt the small hearts. You should use the same pattern of small hearts, then the back will have pretty collections of small hearts in pastel shades. I think it would be beautiful on the back and the quilting wouldn't overpower the dark hearts on the front.