Not gonna comment on this thread because my answers have already been represented by others. Or...I'd just try to summarize what others have said thus far. (as I just like doing this >u<")
+ and - on having arguments IRL:
+) You can see other's expression during pointing out arguments. In psychology you can detect other's facial expression during this moment to figure out whether they're lying or the vice versa (although then lying has two definite purpose, in positive way or negative way).
+) Could be mostly preferred in order to raise quality in arguments itself. Would be better if you have solid evidence in stating your opinion in debates.
+) Preferably done with close friends who understands your points.
+) Authority/leader roles can be important if they're trustworthy enough.
-) Can lead to verbal and/or - far worse - physical fights if not done correctly and in the most respectable way.
-) If you don't have any kind of datas to provide your arguments, then there's a chance it'll lead into wrong way. Thus, you gotta be skeptical when they don't provide this kind of data, except when the fact's really obvious.
-) Possible short thinking time when you're pressured by the time limit.
-) False in grammar usages can lead to misunderstanding if you're having one with foreigners.
+ and - on having arguments in internet:
+) Anonymity, so it's safer when you wish to stay away from your real identity or socials in discussing things.
+) Everyone can state their opinions if they're not brave enough to do it IRL.
+) Plenty of time to look upon evidence to support your arguments, as well as time to post it (but not too long though, especially with 7-days of limitation to bump certain thread).
+) Can refine your sentences to make it in perfect order before presenting it in a form of a post.
+) No physical fights involved when things are getting off from topic, although your mental might be prone to this if you're not strong enough.
-) Impossible to see your respondent's facial expression so psychologically it's rather hard to detect the truth from their arguments (although eventually total solid evidence will tell everything to you).
-) The validity of one smack and talk kind of debates might be questionable if there's no further research.
-) Some sources around the internet itself can be also questioned, especially from blogs if they don't provide worth evidences.
-) False in grammar usages can lead to misunderstanding if you're having one with foreigners.
Thus, giving me some reasons as to why I prefer internet arguments over IRL ones. But hey, it's just from my introvert side and I prefer to keep everything I want to say in order when possible via writings rather than verbal.
Keep in mind there might be more + and - I haven't included up there yet, so yep.
Also Salt, there might be some authorities out there that
had spread some lies to public in order to protect themselves out of danger. Mind you this is just an example; some are known and discovered, some aren't. And the comfortable side of lying can only last in short time, I believe.