Will we need a car when we are at the conference? ;-)
I went to Laguna last year and a car was not needed. They have shuttle buses that will take you to and from where you need to go. I had a car because I went on vacation the week before. I really didn't need it when I was there, but it came in handy when I drove to the Laguna Art School even though there was a bus that went there. I packed in a few other people with me. The resort had a ridiculous overnight parking fee. I decided to just bite my tongue and pay for the car since it was convenient for the entire time of my vacation/conference. I don't regret having the car, but it is not needed. You will be busy the entire time you are there.
One older couple mentioned that they went the previous year taking the bus. Because they took plein air workshops together, they found that they could come and go at their own pace when having their own car.
To help you decide, find out if there is a parking fee at the hotel and event discount. Find out if you can get a rental from the hotel for days prior or after the event. Consider if you can get a shuttle or transportation to and from the airport. Befriend someone who has a car.
You say we - are you going with a friend who might split the costs?