You ride whatever you are comforatable with. Sportives are all about personal challenges whether that's just completing the event, or beating your previous time for that course or distance.
A group of us did our first Sportive back in April, the Scottish Bike Show Sportive 65 miles and 3 of our group did so on hardtails! They were a little slower than those of us on road bikes but they completed it and thouroughly enjoyed it.