Single or Tandem
Do you plan to paddle solo or tandem? This is one of the most basic questions you'll have to answer. While one person can paddle a tandem alone, it requires sitting in the rear of the kayak while ballasting the front. The kayak will move, but not at its optimal level. On the other hand, it's a lot of fun to go out with a partner, often safer, and usually cheaper than buying two boats. There are a few tandem sit-on-tops that have a jump seat between the front and rear seat wells. This seat arrangement makes it possible to balance weight for better performance when paddling alone, and may be a good option to try if you want to paddle both tandem and solo.
Your body determines how the kayak will perform. You probably wouldn't buy new pants without trying them on first. The same rule applies to kayaks. When you test paddle, you aren't so much looking for mechanical failure as you are trying to get sense of how the kayak fits. It goes beyond just height and weight-people carry weight and proportions in different ways, and these differences translate into how you balance in a kayak. You can always learn how to work with different kinds of kayaks-experts tend to balance better than beginners-but know that it will take time and practice, especially if you decide on a specialized kayak.