I like both. I currently have the Curve because I had to pay full price since T-mobile isn't fun. But I still like my Curve. If you really want to know, kinda hint by asking her which one she likes better.
I don't think that the bold is a "manly" phone its really about what the person wants. Does your gf like bigger phones or does she perfer a smaller sleek sexy phone. Either way I don't think you can go wrong they are both BBs. I said get her the 8900 more of a "girlish" phone.