I use Tapatalk versus the "official" forum app.
The main reason was because, at the time, I could increase the font in Tapatalk (which I couldn't do in the other app). And, I could have one app for all the forums I visit versus three or four apps/icons.
Now, I understand the forum app allows you to increase the font. But, I'm accustomed to Tapatalk, so that's where I stay.
Both are good apps - guess its just what works for you.
Marilyn