What kind domain should I choose?

[eyeglasses]

I am planning to desing a glasses related site, I know it is very important to choose a good domain, some peoples say we shoud register a domain with our brand in, some say we shoud register one with our main product name in the domain, and I also found zennie, an eyeglasses seller, with their brand in the domain, and prescriptionglassesonline.net, with their product name in the domain, both are very successful, what do you think?