Ditto what Chikabowa said. When it comes to exposure, both lenses will transmit the same amount of light to the sensor if they're both set to the same aperture and they both will have the same depth of field at that aperture if everything else is the same (same camera sensor size, same distance to subject, same focal length, etc.). However, they the sharpness/clarity and contrast of the image may be different, because of other factors. lenses tend to be at their sharpest a few stops down from their maximum aperture. So, the lens that opens up to 1.8 may be sharper at 2.8 than the lens with a max aperture of 2.8 would be. The quality of the glass elements, the number of glass elements, optical coatings, build quality, etc.. may be different between lenses and those can have an effect on the image.
So, in terms of overall exposure, no difference. In terms of image quality, possible difference. Will you notice the difference in image quality? Hard to say without looking at actual samples from both lenses. You might wanna check out image samples by doing google searches. Also, on some image sharing sites, like Flickr, you can do searches by specific equipment to see images with those lenses.
