I don't bring both, I only bring the hard logbook. I use Logbook Pro mainly for my own backup. Plus, it makes figuring out recency times (past 90 days, past 6 months, etc.) much easier and faster... only takes a click of a button. Also, It'll pick up on any errors in your paper book. After you enter all your info into Logbook Pro, if the times don't match what's in your paper logbook (this happened to me), You probably made a mistake somewhere with your math. I had a "Page Total" that had an hour extra multi than what I actually had on that page. Logbook Pro picked up on that and I was able to correct it.

I also use it if someone wants to see the last few pages of my logbook. Instead of making copies of my book and faxing them, I use Logbook Pro to output the pages in .pdf format and email them.

"The pdf with your flight log is really quite nice for reviewing the data. No need to bother with anything else."

Quote from one of the recruiters that I sent said .pdf files to.