Attention from other girls is always a good thing.
From my personal experience, I think you should leave the shy girl and go with the good looking one, unless you want to have a relationship with her and bad sex. I may be prejudice, but this is what I think about shy and timid girls:
1. They are just as timid and insecure in bed, turning the lights off so that you can't see their body, insisting to have sex under the blanket, only in the missionary position, never going down on you etc... The list goes on.
2. Girls like that, as hard-to-get as she seems to be, usually have an extremely weird idea on how a relationship works, as if the man is the one who should give, give, give and not get anything in return. If your girl fits my preconception, she won't let you into her pants until you have "proven your love" by walking through hell and high water.
Good looking girls on the other hand are usually much more confident and experienced in this area, and know that a relationship is about give and take.