I have a house in Placencia so have done both. Flying is quick and the views are great, however the drive is nicer and a car gives you the flexibility to explore on the way. It also makes it easy to get around in the Placencia area. You will most likely end up renting a golf cart while there anyway so might as well get a car from Belize City. The cost will be the same as flying (including your gas) plus the freedom gained! On the drive there is the must see Belize zoo, the inland blue hole, Marie Sharp's hot sauce factory and more. The road is paved all the way and is almost all in quite good condition as there was a lot of maintenance done this year. Mind the speed bumps! They are all over and meant to slow traffic to keep pedestrians, motorcyclists and bicycle riders safe. It is a 3 hour drive. There are a few single lane bridges too. Keep safe and only pass when you are sure it's safe. Locals will sometimes pass anytime! I still recommend driving. Additionally, you can then stay late in town and back to Robert's Grove whenever you want. No shuttle waiting or calling!