So is it better to wet the tooth brush or leave it dry when brushing to prevent cavities/caries?

Either: Since once it goes in your mouth it gets wet, it really doesn' t matter. Some people like to rinse their brush under hot water to soften the bristles first. Ne need for this if you are using a soft brush, which is recommended. Hard brushes are best used for cleaning golf clubs, bathroom tile grout, and the grooves on the soles of your shoes.

Toothbrush: This is a personal preference. If you wet it the toothpaste will foam more. If your mouth is dry this may improve the lubrication until your saliva becomes stimulated. If you wet it too much you will just have to spit it back out as your mouth fills with the foam from your toothbrush.