For the most part the components on the bike are made by 3 companies (SRAM, Shimano, Campy) and the frames are mostly made in a couple factories in China or Taiwan. The frame designs may be slightly different from brand to brand, but for the most part the biggest difference between an Aluminum X and and Aluminum Y or a CF A and a CF B is paint and logos.

I'd say buy based on the best components and fit you can get for your particular price point.