I would say there are a few fundamental differences between the careers, they often seem to be lumped together as being similar, and I whilst they are to some degree, they are big differences as well.
Obviously one of the main ones is your patients - vets treat animals (but also have to deal with people, so not a option if you don't like people, as some people seem to think!) and doctors treat humans. So which can you see yourself wanting to treat/work with? Some people prefer the fact there seems to be more 'worth' in treating people to animals, other people couldn't deal with operating on a person etc.
Vets tend to end up being more of a jack of all trades. yes, there is chance to work in one field more, specialise etc, but...vets will be consultants, surgeons, doctors, give health advice, dentists, podiatrists and so on. Whereas doctors will end up more limited in the areas they work in, but are very skilled in that area. Again, which would you prefer?
There are other differences as well, but it is really down to where you can see yourself being. Have you done any work experience yet? Try and arrange a week with a vet and a week with a doctor. Hopefully that will help you lean towards one or the other. How about other work experience. Which would you prefer to do, a week on a farm covered in cow poo, kicked in the shins by numerous cows, working outside in the cold, dark and pouring rain. Or in an old peoples home and all that entails, bathing people, dealing with catheters, dealing with people dieing etc. I would have thought pretty soon you would realise what area you prefer working in!!