The sit-on-top vs. sit-inside choice shapes almost everything else about how a kayak handles and what it's actually good for — it's worth settling this before comparing length, material, or price.
Sit-on-top vs. sit-inside
Sit-on-top kayaks have a sealed hull with the paddler sitting on top rather than inside, with drainage scupper holes that let any water that splashes on deck drain back out automatically. They're generally more stable, easier to get on and off (including re-entering from the water), and better suited to warm-water use, fishing, and beginners — the tradeoff is a wetter ride, since there's no enclosed cockpit keeping splash off the paddler.
Sit-inside kayaks have an enclosed cockpit that the paddler sits inside, keeping more spray and splash off the body, and generally offering a more efficient, faster paddling position for covering distance. They can feel less stable to a beginner, since the boat sits lower and narrower in the water, and getting back in from the water after a capsize is a genuinely more involved skill than with a sit-on-top design.
Length is a speed-vs-maneuverability tradeoff
Longer kayaks track straighter and move more efficiently through the water over distance, favoring open water and touring. Shorter kayaks turn more easily and maneuver better in tighter spaces, favoring rivers, creeks, and more technical paddling. Neither is universally better — it depends on the kind of water you'll actually paddle most.
Material affects weight, durability, and price together
Rotomolded plastic is the most affordable and impact-resistant, but the heaviest to carry and load. Thermoformed plastic is lighter and often better looking, at a moderate price increase, with good but somewhat less impact resistance than rotomolded. Composite materials (fiberglass, carbon fiber) are the lightest and often the best-performing, at a significantly higher price and generally less impact tolerance for rough handling or rocky put-ins.
The one thing people forget
Check the kayak's actual weight against your realistic ability to load and unload it alone, especially onto a vehicle roof — a kayak that performs beautifully on the water is much less useful if it's consistently difficult or physically demanding to transport by yourself before ever reaching the water.