In short, 25GB blu-ray discs are single layer and 50GB blu-ray discs are dual layer. The difference in the size will be due to several factors: bit rate, length of film, extras included, alternate audio, alternate scenes, 3D, etc.
That being said, there will not be a difference in the quality of a film if you can find it on both single and dual layer blu-ray. For example, a standard edition film may fit on a single layer blu-ray but a special edition with extra features or 3D may require a larger capacity dual layer disc. The film will be the same.
For the PC vs. stand alone blu-ray players you will find very different opinions. A standalone blu-ray player usually has decent picture quality out of the box. A PC blu-ray player may...