There are a few key considerations to be made when choosing between a sit on-top or a sit inside kayak.
Sit-on top kayaks are suited more for warmer weather. Sit on-tops typical weight capacity is around 275 pounds. Sit-on tops are meant to take on water and they are designed for a wet ride.
Sit inside are for year round usage. It is somewhat more difficult to get back in a sit-inside kayak in the event of a roll. Sit on tops, on the other hand are easier to get back into in case of a roll or if you decide to take a swim .
Performance wise, both are fairly equivalent as far as performance.
The key deciding factor is really which the time of year you plan to kayak. Sit inside kayaks a re more suited for year-round usage.
Watch the following video for further explanation.