The XFX 280 Core edition is a base model, the Gigabyte 280 OC's the overclocked version, so it should be roughly 10% faster. Both companies provide a Core model and an Overclocked, or Black Edition version.
The XFX fan size is roughly 85mm, however the size really doesn't matter, what matters is the air displacement which leads to heat displacement. Both models should be adequately designed for heat management, so the stock fans is not really a major concern. As long as the cards are running under 95'C, they are fine, anything in the range of 55-75'C is acceptable.