Purchasing Excursions Tickets - Online or Onboard Ship?

Are excursion tickets cheaper to buy beforehand on Carnival's website or onboard the ship?  Also, has anyone been unable to buy tickets onboard because they were already sold out?  I've heard that waiting until you're onboard the ship is the way to go, but I wanted to see what other people thought.  Thanks so much! :)

Same price whether before or on the ship. the popular tours can sell out prior to the cruise. if you wait, you may miss out on the one you want. once your trip is paid in full i would purchase the ones you want. and for the other that your not sure what you want to do then wait till your on the ship. alot of times the excursion desk is very busy unless its a port day and then you may be able to go right up to the desk :)

Same price as was already stated.  One advantage to booking beforehand means you do not need to stand in line once on the ship to book your excursion.  LMP

I like to book my excursion on line ahead because that is one more thing I have paid for before I go, and I don't have a big end of the cruise bill.

We also prefer to book our excursions on line (through Carnival) ahead of time. For us, it's an expense  that's paid off long before we sail, we know we've gotten the excursions we wanted (not sold out!), and the tickets are always delivered to our cabin so we don't risk losing them in the hustle and bustle of packing. Now there are SOME tours you can't book ahead of time - when we booked the Everglades tour for after returning to Miami, we had to do that at the Excursion desk once we sailed. To us, going online and purchasing the excursions we want works well for us - we've even had a couple we were sailing with come over once and had a small "excursion booking" party!  I haven't run into being "shut out" either on line or at the Excursion desk but I have talked to people on the cruises who HAVE waited to book and found out the excursions they wanted were all booked up.