I usually find straight hair more attractive, but it really depends on the girl. Some girls look really great with curly hair, but not so great with straight hair. So it really depends on who's wearing it and how.
I have naturally pin straight hair. I wish it was curly. I can't even get mine to curl with a hot curling iron - like it stays for 10 minutes then starts flattening out again even if I spray it like crazy.