Pegasus Airlines: $71 

Turkish Airlines: $624 

Is there something I don´t know that would make me regret it?

Turkish Airlines is generally more pricy than Pegasus. I always use Pegasus and I am happy with them so far. There seem big difference between those two prices you mentioned above, however it may be campaign or something else. 

If it is short distance then go for the cheaper one.

We flew from Scotland to Istanbul just over a week ago with Turkish Airlines. It cost us $362 dollars each for the 4 hour each way return flight. We often use Pegasus as well.