A lighter bike will definately be more fun, and a bit faster on the hills. You may want to hold on purchasing a lightweight, till you are about 16. I bought my first racer at 14, and had to get a size too big for me. Its OK for size now.
Im also an ultra-light body type, about 130lbs, 5'10. I found that swimming was good for upper body strength and for keeping well stretched, something that cycling just doesnt do.