As a rule, it's best to use the in-client report system if you can. The IFS is what is tracking and issuing the vast majority of player's punishments, and the fastest way for a player to be punished is for them to be reported after a match. While it may take more than one game to bring the account up to a punishment level, the only way to get proper documentation of what the player has been doing, is by using the in-client reports.
With a ticket, it will go to a Support Staff, like the others have already said. But from there, we don't ever know what happens to those ticket reports. Perhaps the players are punished if they have a history of reports. Perhaps they are pardoned and the ticket closed out, as they have no history of reports. I'm sure there's some type of documentation that a report was submitted, but I don't think anyone has ever clarified how it interacts with reports submitted to the IFS directly.
I tend to agree with Kei, and only use ticket reports when it is something that cannot be submitted as an in-client report.