Well I love pink so i'm partial to the pink. And it makes Lexi look more girly. I'm the type of person that buys blue for the boys and pink for the girls. (except with my male dog. He has a hot pink collar. But he looks good in pink)
But I do like the green too. It really pops on her.
I think you should do both green and pink. They look really good together and on your horse and I find 2 colours is brighter. You could get like a pink saddle pad and green polos or just whatever you can get in those colours:)