Sometimes both can work, sometimes only one will work. Like everyone said, it depends on the champion and the playstyle.

Keep in mind that Flash has a remedial purpose, in a sense that it allows you to correct bad positioning, which can be quite a problem for new players. Also, for a number of champion it means the difference between having an escape option and certain death.

But once you can position yourself consistently, then you can utilise the fact that some champions do tend to work better with Ghost than Flash. Burbinator has a good post already on this.

Depends on your champ. There are some champs that ghost just makes perfect sense: Olaf, Shen, Singed, Poppy, and yi? These are champs with abilities that being able to chase/continuously hit with said abilities is crucial for winning. Olaf's ult + axe, shen's taunt, poppy's burst + stampede, singed's poison, and Yi's fast attack + movement speed.

Other champs with burst or one time abilities I would choose flash.

It all depends on the champ you're playing.