Depends on what you’re planning to do- race the boat; leave it in the water with minimal maintenance; sail it seasonally? Each will have its own trade offs. The best choice also depends on where you keep the boat, in terms of water temperatures, salinity, and nutrient levels. Generally an ablative is a good safe choice for many uses, but not always.
Sanding off what’s there may also prove to be more work than you expect. Sanding bottom paint is not easy or fun work, and good PPE is advisable. Depending on the condition of the existing paint, and whether it’s compatible with what you want to use going forward, sanding it all off may or may not be best.