[Catalyst] To plugin or not to plugin; style/best-practices

[apv]

I'm working on a user supplied comment. I want to apply some auto 
formatting to it (paras but with somewhat complicated rules for where 
they go, no simple splitting on "\n\n," so it's not going to be very 
compact code) in addition to HTML sanitizing.

Should this be a plugin or a controller? If a plugin makes sense, is 
there one already out there I should look at for good learning code?

-Ashley