I get this question a lot. Often someone calls who is planning to fly to Belize, and then transfer somewhere else like Dangriga, Hopkins, or Placencia. In most cases I advise them to fly.
First of all, you can only rent and return cars in Belize City, so if you rent it at the beginning you will be committed to hang on to it for the rest of your trip (assuming you are not planning to spend any more time in the city than you have to).
It can be expensive to rent a car in Belize. The cheapest car I have found on-line comes out to about $60/day with tax and insurance, and don’t forget that gas costs about $6.00 US per gallon!
Second, most flights to Belize land around 2 – 4 pm, and you have to get through customs and immigration, which takes a minimum of 30 minutes…. which means you will be picking up your rental car late in the day and driving in an unfamiliar country at night. Not recommended.
Also, most guests want to take tours in Belize almost every day, and transportation is always included. So you are paying at least $60/day for your car to sit there.
Finally, it can be dangerous to drive in Belize. Some Belizean drivers drive recklessly, the roads are often in poor shape, and you are not familiar with local driving customs. For example, most of Belize City is one-way roads, but they are not always marked as such.
There are a few instances where it makes good sense to drive. A party of 4+ might really save money by driving. If you plan to spend a lot of time touring around on the roads, obviously you would need a vehicle. But think hard about it before you decide to drive. A flight down to Dangriga (about 3 hours from Belize City by road) takes 20 minutes at a cost of $125 US/person round trip.
One final point, if you fly instead of drive, you get a scenic flight in the bargain!