what i think is most of the 75-300/70-300mm lens are ranged between F/4-5.6. The 70-200 are F/2.8, better quality basically. I did used a Sigma 70-300 lens before, and eventually upgraded to a 70-200mm. Heavier, but gives you sharpness and good contrast.
Totally different animals. A lot of manufacturers make bright, fast focussing lenses in the 70-200 range, i.e. Canon 70-200/2.8L IS, Nikon 70-200/2.8VR, Sigma 70-200/2.8 EX HSM. These tend to be pro level, well built, and expensive.
OTOH, the 70/75-300 range is populated by cheaper, darker lenses with lower build quality. There are exceptions of course, but the longer telephoto is not everything.