Whether one is better than the other, depends on your preferences. As others mentioned, there's no significant difference in overall processing time.
Online filing --
Get almost-immediate receipt notice
You have control over how your evidence is digitally recorded
Web form could be glitchy, confusing, have incomplete instructions
Paper filing --
No need to deal with the quirks of the web form
No file size limit for the evidence you want to submit
Could get lost in the mail or some documents could get lost at USCIS