How many times have you gone snowboarding? Over time, I'm sure you will develop confidence and lose your fear of speed. To me, the park is a hell of a lot scarier than speed! I wouldn't purchase a board in the interest of minimizing speed - you ride the board after all, it doesn't ride you!
If you are sure that you will be spending most of your time in the park, get the shorter board. Otherwise get the longer board. Remember that you don't choose board length based on your height - you choose board length based on weight, riding style, and personal preference. For reference - I weigh 105 pounds. My first board was a 143 and my new board is a 145.
Are you stuck with such a small selection of sizes because you are buying from a store? Have you considered looking online? Perhaps you'll find a better selection there. If you don't mind me asking, which K2 are you considering?